Comments do that are usually swiftly and correctly flagged by users (or mods). In egregious cases, or repeated cases, we warn and/or ban the account.
If you see cases where this is not true, the most likely reason is that we haven't seen them yet. (We don't see everything that gets posted to HN, or even everything that gets posted to a large thread.) The thing to do then is flag the comment (https://news.ycombinator.com/newsfaq.html#cflag) and/or to email us at hn@ycombinator.com in egregious cases.
Alternatively, since different people have different thresholds for considering something to be antisemitic, there may often be things that are not flagged that others think should be, and vice versa. It's difficult to determine what the right threshold should be, but it often won't match the preference of the most sensitive users. If productive conversation is to happen, it may require some degree of discomfort.
If you see cases where this is not true, the most likely reason is that we haven't seen them yet. (We don't see everything that gets posted to HN, or even everything that gets posted to a large thread.) The thing to do then is flag the comment (https://news.ycombinator.com/newsfaq.html#cflag) and/or to email us at hn@ycombinator.com in egregious cases.