It's why I prefer not to get bogged down in litigation over what counts as social media. It's far more productive to look at the individual mechanisms that make web platforms bad for socialising.

Agreed. For me, "social media" is just any website/app that allows users to contribute content and make comments.

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).