Yes, they're different things.
Of course, I only use "six-year-old" as shorthand. I'm not six; I'm twenty-seven. An autistic twenty-seven-year-old is not a six-year-old any more than a twelve-year-old dyslexic who "reads at a first-grade level" uses the same reading strategy as a neurotypical first-grader. But, as far as shorthand goes, it works well enough, at least for small areas of skill: What I can do with emotional control is about as much as a six-year-old can do.
So what does this mean for me in real life?
The most obvious effect to others is that I over-express emotion. The idiom "wearing her heart on her sleeve" applies quite well to me. When I feel something, I express it obviously and strongly--I hardly suppress emotional expression at all. I will cry almost immediately if I feel sad or frustrated; I will immediately express anger, with very little inhibition. If I am grateful to someone, I will express that very strongly as well. What I feel tends to be extremely obvious to the NTs around me because, while I have learned how to express emotion, I have not yet learned how to express levels of emotion. Oddly enough, this can result in misreading by NTs who believe I am feeling emotions more strongly than I actually am.
(I believe this may be part of the reason for the misdiagnosis of many autistic females as having Borderline Personality Disorder. For BPD, the emotional extremes are generally real extremes rather than overexpressed, and it can be difficult to tell the difference, even for the woman herself, because of course she has no experience of NT emotions. The only way to tell them apart is to figure out whether your emotional expression is strong even when your felt emotion is not particularly overwhelming.)
This isn't the only way that autism can interact with emotional expression. Some autistics don't express much emotion. (This "flat affect" is also seen with people who have schizophrenia and related disorders, as well as the movement disorders lumped in with schizophrenia, and was part of the reason autism was at first mistaken for part of the schizophrenia family). It can be related to not being too good at reading others' emotions and not copying them. It can be related to a tendency not to communicate emotion just like you may not communicate other things. It can be a matter of being so overwhelmed by emotion that attempting to communicate it is too strenuous a task.
Inconsistent emotional expression is also related to autism. We find ourselves laughing at funerals or when others suffer a misfortune, even though we have no desire to hurt anyone, nor do these thigns genuinely make us happy. We may find it difficult to show the proper emotion--difficult to "copy" the emotions of others around us, so that we may find a comedy quite entertaining, but laugh as little in the company of others as we would if we were watching the comedy alone. For some autistics, laughter is not infectious. Most of us, to some degree, are less likely to mimic the emotions of those around us. (And some of us mimic the emotions of others too much! The phenomenon of extremely empathic autistics is known in the autism community but has had little actual research. I have even heard some reports of people whose sense of empathy "switches on" when they are not trying to use language.)
So here I have this problem: I express emotion strongly, whether I like it or not; and people around me don't realize this. As a child, I was often accused of being "overly dramatic". I stil have to warn counselors and advisors that the emotion I express is actually not nearly as extreme as it seems. And I have such a hard time hiding what I am feeling that the only way I can prevent someone from figuring out what I am feeling is to physically distance myself from them. It's very much like having an on-off switch for emotional expression: Either I express it fully, or not at all. Unlike most people's continuous spectra of emotional signals, I have only a step function--one or zero, on or off.
I am about as good as a six-year-old at inhibiting emotional expression: That is to say, not very good at all. I can tamp down the volume; I can delay expressing emotion, but the control is shaky, and I still have tantrums with fair regularity. What I have that most six-year-olds don't have, though, is an adult's ability to understand and work around my own weaknesses.
As a child, I was frequently berated for not being able to control myself. Like many of my other difficulties, it was assumed that I did this because I did not want to or because I was spoiled; inability was confused with unwillingness. I believe this was the wrong approach. Just like it is not appropriate to punish a two-year-old for refusing to tie his shoes, it is not appropriate to assume that an autistic person who has problems controlling emotional expression must be willfully allowing himself to get out of control. It is entirely possible that, like me, they are simply unable to control themselves to that degree.
Insisting on control I didn't have was a bad way to approach my problem. By six, I was surreptitiously hurting myself to deal with things. By fourteen, I had "graduated" to an ongoing self-injury problem that, by the age of twenty, resulted in hospitalization. I was so desperate to control myself that I used my own body's fight-or-flight response to physical injury to try to force control of emotion.
Just punishing lack of control is pretty much useless; but there are alternative approaches. There's no reason to throw up one's hands and say, "Oh, I can't control myself; so the world will just have to deal with my tantrums, outbursts, crying fits, and other such events. It's not my fault."
It's true that it may not be your fault that you can't control emotional expression. However, you can control just about everything else. You can control whether or not you apologize. You can usually control whether you remove yourself from the situation before you offend or hurt anyone. You can control your explanations to the people who know you: If they know that you aren't nearly as sad or angry as you seem, they can change their interpretation of your actions. You can control the responsibility you take for what you've done--making reparations, making repairs, making apologies.
And you can also control your stress level. You know what causes meltdowns and emotional outbursts for you; you can plan for those things. For me, disappointment is a big issue because it forces me to quickly change my mental landscape of the future, leaving me feeling very unbalanced until I have a new plan. If I am looking forward to something, I can plan by thinking of alternatives: "If the meeting is canceled, I'll go to the cafeteria, get some coffee, and do some homework. If the cafeteria is closed, I'll go to the coffee shop instead. If I can't do that because I've forgotten money or my homework, I'll go back home and do the homework on the table near the window." I'm not nearly as badly thrown off balance when I have alternatives for every situation.
Getting out of the situation is also a great way to deal with these issues. If you are getting out of control and you know it, you can enact a procedure involving distancing yourself--physically, if possible. Public restrooms can be a good last-resort bolt-hole for the desperate, but it's best to have a place of your own in any location where you spend a great deal of time. If you live with family, having a room of your own and declaring it inviolate is practically a necessity. At school, if I really need to prevent something embarrassing (especially full-out meltdown--which feels every bit as extreme as it looks, unlike many examples of my emotional expression), I can go to the disability services office, where they are much less likely to get out the tasers!
Kids often have a hard time getting out of the situations that create problems for them. Their locations are often monitored and they're required to be in some particular place or another--being in a classroom can be as good as being caught in a trap. And they also have less maturity and less self-awareness so that they are less likely to be able to predict problems before they happen.
I've mentioned meltdowns before. Are they an example of emotional overexpression? No; but they're related. A meltdown isn't overexpressed emotion; it feels every bit as bad as it looks. When you're truly out of control--when you're not thinking or communicating--you're pretty much expressing as much emotional distress as it's possible to express anyway. But a lack of emotional control can lead to meltdowns because of the simple discomfort of being sad or angry and because of others' reactions to things they think are extreme emotions. As a child, for example, I would cry at only slight provocation. My parents would then yell at me for being immature, and my shame and the pain caused by auditory hypersensitivity would eventually catapult me into meltdown. Emotion is one stressor that can add to the load and eventually, togther with other stressors, lead to a meltdown.
Now that I'm an adult, I know that I have less control over my emotions than other people do; and I've learned that it's best to keep strongly expressed emotions private. I've learned to go somewhere by myself when necessary. I know that strong emotions make other people uncomfortable because they cannot help echoing those feelings themselves; so, like any private business, they should be kept behind closed doors.
That's not to say that I suppress emotion. After so many years of trying, I know better. Emotion is a healthy part of who I am; the problem is that I express it so strongly, not that I feel what I assume is approximately the same level of emotion as the average person does. Since strongly expressed emotion can lead to miscommunication (and just by itself interfere with my ability to communicate and relate to others), I have learned that interaction is best done when I am calm and not feeling particularly strong emotion. That way I can concentrate on passing facts from person to person, rather than being hampered by emotional interference.
So here's my suggestion to those dealing with their own, or their childrens', emotional overexpression: Don't shame yourself or your child for expressing emotion strongly. Instead, make it a goal to learn to understand feelings, predict when they will affect stress levels, reduce the stress level when necessary by leaving the situation or using other stress-management techniques, and responsibly deal with the results of any emotional outburst that still evades all efforts at management.