I guarantee you Tumblr posts like the one you mentioned are more common than you might think..
Speaking on my behalf, I remember seeing many posts back then glorifying and romanticizing depression, eating disorders (anorexia and bulimia), overdosing, suicidal ideation, self-harming. This came about through my interest in post-hardcore bands, the general community surrounding the bands weren't all that happy with life so there were other communities to identify with, let's just say that.. early 2010's emo/scene kids were in deep with mental health issues, myself included.
There's the people that needed an outlet by making black and white image posts with poetry.. (If you were to Google "tumblr depression", and go to Images.. that's exactly what you'd find.) Then there's the people that identified with the content and its general vibes. It was never really brought into question whether or not it was fake- everyone was so focused on needing to know that they weren't alone in their struggles that its legitimacy didn't matter. They were being understood anyways and seeing other users relate to it. The authenticity was found in everyone else's reactions.
All of the topics I mentioned have since been sorta blacklisted on Tumblr with the exception of specific tags that get past the blacklisting. Essentially, a lot of the content was tolerated on the platform but hidden away, as opposed to having it all be removed. Unfortunately, as I understand it, all this stuff is apparently easy to find on TikTok as well.
The fact that those communities have had a resurgence there does not surprise me in the slightest. If there's another platform that ends up becoming as big as TikTok, there's a good chance this stuff will show up there too, just slipping by under the radar. Bypassing any active measures that were taken to ensure that the topics wouldn't appear again.
This is exactly why I don't think banning TikTok would do much. The underlying sources of the mental health issues you're concerned about aren't gonna be fixed by deleting some app and/or changing the way it works