I took a look at that along with the reply we detached, which was https://news.ycombinator.com/item?id=15086980, and it seems to me a fair place to draw the line. The original comment was predominantly thoughtful while the detached comment was predominantly flamey.
These calls are somewhat arbitrary because when a thread degenerates, you can always trace the root of the degeneration further upthread. But moderation is more like pruning a tree than excising a tumor. The goal isn't to get rid of all malignancy—that would be too draconian and is impossible anyway. It's to detach the bulk of unwanted growth.
People can reasonably differ about individual cases, but the principle here ought to be pretty solid for anyone who has read https://news.ycombinator.com/newsguidelines.html and wants to use the site as intended.
The point is that if there is a bias, it would be that moderation seems to target those that disagree when people bring up sexism and racism in unrelated article discussions. It seems very safe, even profitable in terms of votes to bring up sexism and racism, which would be contrary to the views of matt4077.
I predict that if moderation was a bit more consistent in keeping out discussion about sexism and racism in unrelated articles (unless exceptional new insight), we would actually see less reader fatigue and lower amount of flagging for explicit articles about sexism and racism.
(Just to be extra clear, I don't disagree with the moderation itself on the two above examples if they had been the ones bringing in the topic of sexism into those threads).