EDIT to correct: most are not [flagged], but [dead] anyway, so probably manual moderator action or an automated anti-bot measure.
That's why. Boring, bland, etc. That account's M.O. is basically "write a paragraph that says nothing." Fwiw, I do think AI can be indistinguishable from dumb, boring people, but usually those kinds of people won't be on HN.
I agree it doesn't seem obviously AI. The early comments are all in the same writing style and smell human. Lots of strong opinions e.g.
"logged in after years away and had basically the same experience. the feed is just AI slop and engagement bait now, none of it from people I actually followed." [about Facebook]
HN has got a big problem with silently shadowbanning accounts for no obvious reason. Whether it's an attempt to fight bots gone wrong or something else isn't clear. By the very nature of shadowbanning there is no feedback loop that can correct mistake.
I don't think it's clear at all why people do this. I suspect a large amount of it, at least on a site like HN, is just hapless morons who think it's "cool".