That topic hits home.
I got the "You don't look autistic" from my boss last year. He's somehow lucky that he followed up with something even worse* that just caused me to go blank, because I remember vividly that I was about to retort that yeah, and he didn't look stupid, yet here we are (I can be a little rude when caught off-guard. OK, not true: I can be very nice when adequately prepared and focused, but my default setting is "blunt and snarky").
I got another "You don't look autistic" last week from a doctor whom I was seeing to discuss disability status for another condition, to which I replied the stupidest thing ever: "I know, but the wall is too far behing for me to bang my head repeatedly against it, so I understand that you were fooled", and the only way I can explain why I would say such a thing is because she had hurt me badly during the physical exam, seemed fairly incompetent and definitely adverse. Yes, she was taken aback. But she actually opened up after that, so maybe I should try to be rude to people more often
Usually, I just follow up that crappy comment with a simple yet firm "And what exactly is 'autistic' supposed to look like?", because at some point, people have to acknowledge their prejudice, and recognize that they know very little about autism. I found this to be the best way to start that conversation, but it does make them uneasy. Sadly, I'm only able to do that when I'm not in an overly stressful situation, otherwise I just blurt out random things or shut up, flabbergasted.
I'm not sure I'm past the point of being offended by that remark. I know it comes from a place of ignorance rather than meanness, but I'm still annoyed that the people who utter that phrase mistake their ignorance, or incomplete knowledge, of autism, for actual knowledge of it. And I'm especially annoyed when they think they know better than me.
* That fjhggrtgt of a boss followed up with "Well, as long as you don't bring a rifle to work and shoot us all dead, it's OK, though".