I think if social media required logging into a desktop computer or laptop then it would be a lot less impactful. So I think I still disagree. I think smartphones can be bad even if they are limited to largely non-social media like YouTube or TikTok.
It's probably the combination of both. The smartphone itself is innocuous, but increases the availability of the social media.
However if social media wasn't as ubiquitous it'd likely be less of an issue then it is today.
However, if social media wasn't a thing people just wouldn't constantly pull out their phones, hence id blame it instead of the form factor.