No but I do consider reddit to be and yet hacker news is in essence very similar to a specific subreddit.

It's mostly the community (and moderation on HN) that sets it aside.

I think the right way to frame it is that the particular format of a site doesn't necessarily dictate whether or not something is "social media". It's important to look at how people actually use the site[0], as well as whether the voting/ranking/whatever system looks more like a popularity contest or objective moderation. It usually won't be 100% of either of those, but it will certainly lean one way or the other. I think HN leans toward objective moderation much more than Reddit does, even though HN's moderation is certainly not fully objective.

[0] This use can be heavily influenced by how the owners of the site push things, e.g. HN's guidelines and in-house moderation decisions vs. Facebook's algorithmic news feed that chases user engagement above all else.

I think this is a good analogy, though something I've noticed is that as reddit has taken their product direction more towards social media, it seems that it has been harder to maintain quality in smaller discussion subreddits, because popular posts get picked up and injected into non-subscribers' feeds, so the ability to have a subreddit approaching HN's level of conversation is reduced.

Increasingly it seems users have no concept of subreddits at all, and simply consume a singular home feed (I don't actually know what the new user experience of signing up for reddit on the app looks like, but this is my impression), more like the major social media platforms.

I've been using reddit for a long time and still check it, but I've become considerably less engaged as they've moved towards this kind of lowest-common-denominator slop trough feed approach.