Why Are Teens Using TikTok for Mental Health Self-Diagnosis?

Because they are teens. Even adults are falling in the trap of self-diagnosis, it's easy to project yourself in a teen's shoes where they get convinced by peers that some of their quirks or traits are confirmation that they have some sort of mental issue. I'm pretty sure there are also some people with malicious intent who just want to cause harm. Edit: I didn't realise you were linking to an article. I read it and agree with many of their points.

Self diagnosis isn't new. The problem is the algorithm will now put you in a echo chamber where you will be constantly telling you you have this problem based on some small number of interactions you made.