> If autism is the impaired ability to ascribe mental states
First, autism is not the impaired ability to ascribe mental states. Earlier in your post you more accurately described that as a symptom.
There are attempted theories that get more to what autism "is", that it's a heavier weight on bottom-up processing, processing distinct details more and holistic things less. Another (congruent) idea is related to a "narrow tunnel of attention" and to using more of the brain to attend to a more narrow set of stimuli and thoughts. (You can look up monotropism for more info on this idea.)
Secondly, in my experience and others I've read - autistic individuals seem to often do much better at understanding each other. I mean this in two ways: autists are often better at understanding other autists than they are at understanding neurotypicals, but also in many cases autists may be better at understanding other autists than neurotypicals are at understanding autists.
With this framing you could just as well say neurotypicals lack a theory of mind. Largely we don't do that because there are more neurotypicals than autists.
Of course that's not the full reason and it's not fully symmetrical. If there were a 50/50 split in the population then I think autism would be less of a disability for folks in terms of this theory of mind stuff, but it would still be a significant disability for anyone who struggles with all the other consequences of autism.
But it is still relevant that this "theory of mind" stuff may be a misleading way to talk about it; by default I would expect everyone to have an easier time with a theory of minds like their own. And thinking about it this way also helps inform neurotypicals with autistic loved ones that if they expect their loved one to work hard to understand them, they could do the same.