I'm trans and wasn't diagnosed as a kid because i had high masking autism ("girl autism") instead of the "boy autism" they were looking for.

I disagree, I think the anecdote of an adult over 30 years of age being diagnosed is a fair example of under-diagnosis. And since your comment was more on the over-diagnosis side, I think it’s fair to point out. That the visibility and lowered stigma contribute to the over-diagnosing. It can’t be helped. Medical professionals are subject to the same biases of visibility that the rest of us are, even if they should know better.

Also some patients are certainly self-diagnosing based on freely available information, be it valid or not, and sharing their diagnosis as if it was a real one. When others encounter these claims, their instinct typically isn’t to argue or accuse someone of being a fake autist so they update their own mental models with a “this is what autism looks like” and the trend continues.

It is exactly what I am describing. In any test you will have false positives. Then the broader you test the more false positives you get. This was also a thing during Corona in Germany. At the start of the pandemic only people with symptoms should get tested, because with low case number and even a very good test and test procedure you can easily get more false positives than true positives. This is true for every test where true positives are rare. The math is pretty simple here.

Well, there's two things to consider.

One is just how many folks "self diagnose". Rather than a stigma being reduced, it's often held up as a trait of superiority to the "normies", so some folks will assert it. There's a fine line to walk between unfair stigma versus unjustified glorification. The internet is full of this.

Two is that ultimately, there's room for being subjective even among professionals. See the parents of a kid that my kid was friends with. They lamented they got told by 5 psychologists that their kid was not autistic before they finally found one that "correctly" saw the kid's autism. They were so excited to have proof that their kid was one of those autistic folks that are super smart...