I would really not consider Butters rubbing his knuckles together stimming. It’s not that uncommon for people who are nervous to do something as a physical reaction, like laughing, biting your lip, bouncing your leg, biting your nails, peeling your nails, rubbing your thumbs together… that’s not stimming. That’s just a nervous habit. 

As for the social awareness, yeah he may be socially unaware some of the time but that’s incredibly excusable since he’s a kid. And remember, Butters goes out of his way to involve himself in every social thing that he can. He tries to hang out with the guys a lot, keeps up to date about rumors going around the school and generally is enthusiastic and not hesitant at all to greet new people. 

To me that’s the thing that really works against the idea that he has autism, since to at least some degree those with autism tend to be avoidant of social interaction unless their parents make a huge effort to make them social. Stimming itself is usually a way for autistic people to disassociate with things around them and distract themselves with their own noises or something they heard from their favorite show. 

So with Butters, autism doesn’t really add up. There are lots of kids when they’re young that are socially inept or seem to be quirky but that doesn’t mean they’re autistic. It might just mean they’re late bloomers. I think Butters is just one of those kids.