I really would like to know the rationale behind the people trying to flagkill it
Though if I'm being more charitable, most likely some people see this as a political article which shouldnt be on HN (whether you feel that way is probably correlated to your opinion of Black Lives Matter).
If somebody only objects to political articles when it's politics they don't like, I'd be curious as to how they resolve claiming to be against politics while energetically politicizing something.
The topic of the article is fine for HN, in my opinion, despite being political, but that doesn't mean we should settle for a crappy article. Hence, the temptation to flag this one and let a better submission get the focus.
"Notorious" means famous for something seen in a negative light.
It doesn't actually mean that you have the trait in a unusual degree, only that you are particularly well-known and negatively reacted to for the trait.
It may be inaccurate to describe Facebook as "notoriously white", but the fact that they are somewhat less white, proportionally, than the US population doesn't actually demonstrate that. (It might be a basis for arguing that, if they are notoriously white, the notoriety is not deserved, but that's a different issue.)