I totally disagree on both counts: forums are social media, and Lemmy is not a mere forum. Lemmy is a platform where people can create forums, and many of those forums (communities) exist mainly to socialize.
I'll give you that some forums (both on Lemmy and otherwise) that have a clear defined topic - such as tech support for a particular thing - are somewhat different from "social media", but even in those three are often regulars who use the forum to socialize with each other. Any forum with an "off-topic" subforum is social media in my book, in a very real sense (not just technically).
But hey, we can disagree on this and it's fine.
To clarify why I think Lemmy is not a forum: in my eyes, forums are set up by the admins, only the admins can decide which subforums exist and what's allowed in them. Lemmy and reddit are not simple forums because they allow any user to create a subforum and make those choices and decisions, that traditionally are reserved for admins. It's an extremely important difference and makes Lemmy much more of a general social platform and not a focused forum.