I'd argue that it can include HN, but HN is not designed to be nearly as toxic, and actually goes out of the way to be "as not-toxic as possible without breaking the concept of what it's trying to be."
I've spent a while thinking through "What makes social media evil?" - I've read a number of studies, I grew up with the first wave of what we might call "social media" (never used MySpace but was quite familiar with it from friends, extensively used LiveJournal, etc). And my conclusion is that once you go away from "global state" to "individualized algorithmic feeds," it's probably gone evil.
HN, as far as I can tell, has a global state. There may be some replication delays (if it's hosted on multiple servers, which I simply don't know - I can't imagine it requires much in terms of resources to host), but "I see what you see what a non-logged in user sees," with only very, very few differences (flagged/dead posts, mostly). It's not reordering the state based on what it knows about me - it's reordering, globally, for all users, based on the consensus. The same was true of LJ - you had your own feed, but outside permission related differences, if I looked at a community and you did, we both saw the same thing.
This global state also means that there's no "slot machine" behavior of "pull the slider and see what's new." You refresh. It's the same stuff. Maybe two things switched positions. Go away, because it's simply not going to change that fast for you.
HN also doesn't go with the endless scrolling. There's a bottom of the first page. You have to make a deliberate decision to see beyond it, and that's something that historically was just "how things worked," but has been replaced with the bottomless bowl approach. We know that people, given a bottomless bowl of soup, eat more. And given that every ad revenue based social media company has gone to that model, clearly people consume more.
Humans are vulnerable to "metricification" quirks, and you see that with a lot of social medias too. HN only shows your up/down votes, and you can't see anything else. There's a demetrificator extension for it, though I would like the option to turn the metrics off in my profile.
And, finally, HN is "pull" - I have to go to the site, vs it reaching out with notifications to try and drag me back in. Plus, there's the noprocrast setting. It gives you the tools to make it a nuisance to use too much.
So, while HN can be somewhat addictive if you let it, the lack of any "weaponization of psychology" tricks and the very active "No, really, you've been looking at it too long, go away" coding means that I don't consider it particularly toxic, and certainly not in the same style as FB/Twitter/etc.
Why it's a recurring obsession? Probably because a lot of us have been in the tech industry, have seen how it works, have (probably) been addicted at some point, have watched the whole concept go from "Huh, this is kind of a neat way to connect with people I'd never talk to otherwise!" to "Braaaaaaiiiiiinss... via eeeeeyyyyyeeebaaaallllssss!" in the past 10-15 years, and really want no part of it anymore, and want to help ensure other people see it for the vile, human-toxic trash it is.
It'll destroy your ability to focus, make you less emotionally unstable, warp your view of reality in one of a variety of ways, suck basically all the time you let it, try with every trick in the book to addict you. And in exchange, some tech billionaires get another couple feet on their yachts. Yay?