I think things like autism and other mental things are hard to diagnose externally; the self-diagnoses are probably "Yes, I feel the same way" - not so much "I have autism" but "I am similar in some ways". Like identifying as intro- or extroverted - there's self-tests for that (Myers-Briggs or something). But the personality traits in that one are spectrums too - you can be a bit or a lot of all of them.
TL;DR it doesn't really matter much IMO what someone says they are, but it does help if they understand themselves better and know that they're not the only ones with those traits or difficulties in life (if they perceive them as difficulties).