Many things classified today as autism spectrum disorders are hardly incapacitating so I would not call it a mental illness.
It's not physically incapacitating until you kill yourself which is the signal society sends you constantly.
It's pretty incapacitating.
My 5yo boy is officially diagnosed and he is really smart. My father was definitely autistic although never diagnosed.
I was lucky enough to have a false twin brother (11 months younger) that was the most popular guy at school while I was the weirdest (until I started emulating his behavior in social situations).
Today I believe my brain is just wired differently. Some of the most powerful structures in the brain (the GPUs) are devoted analyzing faces and reactions; in my case they are used for other purposes so I had to learn how to process faces using the CPU which is a bit tiresome but works well enough.
Along with hyperfocus, a tendency toward honesty is another such trait. "But people without autism are sometimes honest, too" would be a comparable response.
Some people do just fine, sleeping less. The issue is not listening to your body and waking when you don't want to.
I mean... the oddities that led him to be diagnosed included things like lack of eye contact and bizarre play habits. But, honestly, these are rather unimportant in the grand scheme of things, because the 'normies' were missing pretty fundamental, yet non-diagnosable, personality traits, such as ambition and conscientiousness
What society wants and signals to you constantly is for you to kill yourself because you're not worth anything.
It sucks to say the least.