It's not that far off, frankly. A lot of websites with user comment sections have a social aspect to them. But even if we discount those whose the primary focus is not that, and we take YouTube and IMDb out of it, Reddit and Lemmy lack the sort of stable end result that reference sites and wikis focus on, or the sort of separation between creators and audience that YouTube has (then again isn't Instagram like that too?). Even if we make a distinction here we are still left with a place whose content, curation and discussions are community driven. A subreddit or Lemmy community is nothing but that which the people who participate in it decide. Sounds like a social form of media if I ever seen one. An internet platform for public collective multidirectional communication.
Frankly, I think Reddit and Lemmy resist the classification as "social media" not because there aren't reasons to count them as that, but because the userbases in these sites see it as a dirty word and they like to believe they are above the unwashed, stupid, celebrity-worshipping masses.
They are not. We are not.
How much do we mock people commonly not reading articles and just commenting immediately in both these places? That shows link aggregation is not necessarily the main driving appeal of these places. Even for lurkers and outsiders, they often use Reddit to see what people are commenting, especially as far as advice and recommendations go. We are even in a community whose primary content are questions from other users for us to talk about our opinions and ourselves. How much more socially driven does it need to be for people to accept that it is social media?
Even if one argues it's different from Facebook because we don't use real names, Twitter also goes by pseudonyms and everybody considers that "social media".