(https://news.ycombinator.com/item?id=13094354 is the comment, which I find entirely reasonable but obviously people disagreed, or at least it was considered off-topic)
There is no reason, based on the HN guidelines, that the referenced post should have been downvoted, let alone flagged. Whoever did so abused their power to make such decisions.
It's even more surprising since HN had the "vouch" feature since before Dec. 2016 [0]. My best guess is that people might have reflexively downvoted/flagged upon seeing the opening sentence of "Black people are measurably less likely to own a car or have a bank account".