For what it’s worth, I agree with you about Lemmy and Reddit not really qualifying as “social media.” I think of it more as a spectrum than a binary value…

• Old school forums were very specific to a single topic (though most forums I used did have an “Off Topic” board), and only lightly social – I never knew any forum user outside of their respective forum, and certainly not in real life.
• On the opposite end, Facebook/Insta/TikTok are very social – there’s a lot of expectation that you’ll be interacting with people that you know personally – and they are more “agnostic” (?) of any one particular topic.
• Reddit and Lemmy land somewhere in between those two extremes, in terms of both the social and topical aspects. But neither cross the line into “social media,” at least not for me and my personal definition of the term.

And just to split hairs even a little more, I think Lemmy is more palettable than Reddit for me, by virtue of the smaller (and generally more tech-savvy) user base.