It’s missing enough of the things that characterize social media that it’s unambiguously not in that category.
[edit] however, separately: yes, it should :-)
you can network, dm, and users who post frequently are often recognized here.
you can doom-scroll as much as anywhere else whether it be in the feed or in the comments (so long as you don't have a bias towards different mediums like video).
and also while it might not meet your criteria for social media algorithmically or otherwise, an algorithm is not necessary for someone to engage with the internet in unhealthy ways. the surface area for emotional damage is just different. and that doesn't imply HN is some special place where the only users engaging here are "emotionally well" in both their engagement/usage, and in their personal.
and speaking as someone intimately familiar with relevant behaviors and presentations, the algorithm exerts influence and exploits, but it is ultimately not the RCA -- it's just like a drug dependency for someone with maladaptive coping mechanisms -- the drug isn't "the problem", it's a symptom, and the "drug dealer" is taking advantage of it.
side-thought:
it always makes me wonder if people manufacturing and distributing what i see as a "need" for social media, while employing HOOKED marketing dopamine strategies to establish said need in its users, ever ask themselves: is this a net-good? should i be doing more with my life? is this harmful?
imagine being a demonstrably great mind and seemingly great person, but working somewhere like FB or IG or TikTok and applying every ounce of your intelligence to maximize user engagement.
Personally, IMHO, if we want more people to have fulfilling careers that are a net-gain for society, we need to foster an economy that finds that valuable and will pay for it as such, instead of seemingly, almost intentionally, placing personal fulfillment/net-good for society on the polar opposite end of what pays well.
HN is unambiguously social and unambiguously media.
I get how people arrive at that kind of place—“social” and “media” cover so very much, singly and together, so, what’s not social media?—but if we define it that way, it’s uselessly broad because it doesn’t exclude enough. Also, it includes tons of sites that pre-date (or are similar to sites that pre-date) the term “social media”, and aren’t the kind of thing the term was coined to refer to, like HN. Why even have the term?