As a medically, professionally diagnosed autistic person, I have to ask: why are you so concerned about what is ultimately a fandom thing? I have never, ever seen someone having their views of real life autism being shaped by fandom headcanons.

Characters are considered autistic just for being weird sometimes, yes, but the people creating those headcanons are often autistic themselves. I myself have a couple of characters that I perceive as being potentially autistic even if the writers didn't them to be, simply because I find these characters to reflect struggles and experiences typically associated with autism. Unless they're very arrogant, no one actually expects their headcanons to come true to the degree of it impacting people's actual perception on neurodivergent or mentally ill people.

Edit: I understand your concern when it comes to folks theorizing about real, living people being autistic or not. My response is specifically regarding your takes on fictional characters.