The thing is, the people who are being racist, misogynist, fascist, etc. are also being political. And so their posts will get flagged too.
I think the idea is, basically, that rather than "calling out" these things and getting into a big argument about them each and every time they happen, you just (indicate to the mods that you want to) blow them out of the sky.
How does one contact the moderators to let them know that a comment is racist/sexist/whatever? I don't see any "flag" UI at the comment level, and I don't know who the mods are by name.
If you click on the timestamp of a comment, you should see a 'flag' link. I don't recall, though it might be karma gated. You can also reach the mods via email to hn [at] ycombinator.com.
I think the idea is, basically, that rather than "calling out" these things and getting into a big argument about them each and every time they happen, you just (indicate to the mods that you want to) blow them out of the sky.