The term doesn’t have to mean what the literal words on their own do, or apply to everything that the individual words might, taken in their own.

[edit] and in this case the term is far more useful if it is not as expansive as you suggest. We’d just need another term, then, to describe the kind of thing people usually mean by “social media”.

Yeah, there are lots of other little aspects that push something closer to or farther from being the kind of thing we usual mean by “social media” (“so it’s a fuzzy category?” yes, like approximately all other categories and labels). Downplaying personal or account identity (which HN does to a more extreme degree than even most forums et c) definitely counts as moving something toward the “not social media” side.