When I got my first smartphone almost two decades ago as a 16yo, I used it mostly to play with the system, write my own UIs, debug system daemon issues, tinker with bootloaders and ended up contributing to a community distro for it. It was a fun computer in my pocket that taught me a lot and a massive upgrade over earlier phones where I had to use Opera Mini and Bombus via J2ME.
The vast majority of smartphones of today are toys with very little potential to tinker, and even when it's there, the entry barriers are enormous. Compiling Android is a huge undertaking, while as a teenager, I could already launch vim on my phone screen in a tram and hack on a SMS daemon written in Python. It's incomparable, and even if you do start, you quickly encounter hard barriers like device attestation.
These days I'm not immune to social media either, as evidenced by me writing this on my phone right now - but wouldn't these "worse mental health outcomes" be rather connected with social media usage, media consumption in general, intrusive notifications etc. that are commonly associated with but aren't inherently necessary part of smartphone usage? At the same time, my mental health isn't becoming immune to social media damage when I doomscroll on a PC just because I used a bigger screen with a physical keyboard.