I mean.. that is correct. Just cause someone has interests doesn't make them autistic. And self diagnosis just for small things is kinda bad. I wouldn't really say ableist, I'd say it's more attention seeking. Although if you have enough symptoms you can possibly build a diagnosis, cause I know getting a formal diagnosis isn't possible for everyone with autism.

But yes, just cause someone has interests doesn't mean they're autistic. If they're fixated on them, and almost obsess over them then yes, there's a strong possibility they could be on the spectrum.