In my personal experience, kids and young adults particularly those who grew up immersed in social media (born after ~1995–2000), seem to struggle with recognizing appropriate, undistorted social cues and understanding the real-world consequences of their actions.
To Snapchat harming kids, I think it is more than just evil people doing "five key clusters of harms".
Even adults often expect the same instant reactions and flexible social dynamics found online, which blinds them to the more permanent, harsher outcomes that exist outside of digital spaces.
Anecdotally, the utter shock that shows on some people's face when they realize this is sad, and very disconcerting. (At an extreme think "pranksters", that get shot or punched in the face, and they are confused why that happened, when "everyone loves it online".)
How to fix this? the suggested solutions will not solve this problem, as it does not fit the "clusters of harms".