Even HN has a clout score, and seeing it move up/down, or slapping that up/down arrow, can trigger the same dopamine as social media by MegaCorp.
HN is absolutely "social media", as it Facebook and WhatsApp.
With that said, I do believe social media platforms exist on a spectrum of "mostly benign, maybe even useful" to "mostly harmful" with closed groups/forums well to the benign end, targeted subjects with heavy moderation somewhere to the benign end of the spectrum (HN falls here), and "free for all" well to the "harmful" end (this is where most of Meta lives).