The spectrum of autism isn't mild to disabling.

It's a spectrum of presenting symptoms. What "functional autism" is, is a constellation of symptoms that means you can just about navigate the world as expected by neurotypical people. "Non-functioning" autism or "profound" autism are where the constellation of symptoms makes you unable to function in the world. This is more often than not inclusive of non-verbal autisms and poor motor control autisms.

"Functional" autism can be disabling for the individual in certain conditions - they may be completely unable to read social cues, or be severely impacted by certain stimulatory environments.