I would guess that it's less literally "can't sit still" and more a sign of "poor motor control". Plus I'm pretty sure that scene is meant to show it's just one thing along with "doesn't understand commonly accepted social cues" - "doesn't know where to look", "doesn't understand how to respond to another person", "can't hold a conversation" etc., etc. ("Her reactions are all off" was my attempt at shortening... it was something like "She doesn't know how and when to come in with the next line", which IMO sorta ties in with how Muu-chan just abruptly walks out of the sushi place at the end of that chapter.)
Not to sneer at you or anything, so apologies if it sounds like I'm trying to do that. It's a fair point to raise - I think the mangaka's obviously done some level of research, but I don't want to ignore the possibility she might have got some things "wrong". Plus while I'm not remotely an expert and I'm relatively neurotypical myself, I know enough to be aware a lot of conventional wisdom about this stuff is often thought to be on very shaky grounds, and/or held up as "true" in large part just because "well, that's what we've always believed, so it must be correct". But I still feel like any problems with that sequence are more likely to be me phrasing the English poorly.