While I avoid reading DF whenever possible personally, I'd never flag posts.
I am sure only the moderators have visibility into what's going on here. I would hazard a guess that the user base of HN itself has changed in recent years. It's probably no accident that Gruber points out the ranking shifts in 2021 - a lot of things shifted 2020/2021 due to downstream pandemic impacts.
It's hard to speculate without mod-level visibility though. I'd love for them to weigh in.