It takes some amount of people flagging it before it shows up as [flagged] but flagging submissions affect rankings before it reaches that point. On top of that "Ask HN" posts also have some modifier on them that makes them drop quicker, these two effects seem to be cumulative, explaining why it dropped so quickly

Does HN always tell you when someone flags it? Or just a critical number of users or something like that?

I see the flagged indicator now and then, but I see it somewhat less frequently than I would THINK things get flagged, admittedly that's all conjecture.

If a story gets moderately flagged, it gets deranked but don't show up as "flagged"

Or ideologues.

I for one have never shied away from a "hot headed" argument. Imho, I feel that as adults it is our responsibility to voice our opinions even in times when that opinion will contrast with another. In fact, sometimes it is crucial to do so as often times the most important arguments to be had are the most heated.

I believe the HN esk counter-point would be that HN is not the proper forum for those kinds of discussion. Imho, there is no "proper" forum for these kinds of discussions. They simply need to be had, more now than ever in my lifetime, on every street corner, and in every shop.