Tik Tok is the reason I've been told several times I have ADHD. Pretty sure I don't! This phenomenon is so infuriating. The only half-convincing defense I've heard of it is that it could, maybe, theoretically, disseminate information about illnesses to people who don't have the money to see a professional, but the other effects are so much worse it doesn't seem worth it.
Tiktok and pop-culture generally make me very skeptical on "ADHD", the so-called "spectrum" and "neurodivergence" generally. I'm not saying these things don't exist, but much of it seems very difficult to falsify -- which needs to be the gold standard of determining these sort of things; e.g. "By what process can someone reasonably determine that they are NOT neurodivergent?"
I suspect it's this latter group that social media diagnosis appeals to most - if you're not doing as well as you (or your parents) would like then normally you just have to feel stupid/worthless. These (self) diagnosis provide a level of explanation (and therefore comfort).
I self-diagnosed myself with mild ADD many years ago and even self-medicated (which in terms of productivity worked well). But meeting people with severe ADHD made me reconsider my alleged issues - sure, I can't concentrate for as long as I'd like but these people couldn't sit still for 15 seconds.