You should flag a comment if it's breaking the site guidelines, but it doesn't break the site guidelines to go off topic. Sometimes off-topic comments can be interesting. It's when they become generic tangents that they break the guidelines. If you reread
https://news.ycombinator.com/newsguidelines.html you'll see that that the line is drawn carefully that way.
For example, someone sharing their personal experience about a marginally-related point may be off topic but isn't generic. But turning a thread like this into an argument about, say, the men's rights movement (as happened elsewhere)—that's generic and therefore breaks the guidelines.
A less clear case is the subthread right here that goes into a long discussion about birthdays and Jehovah's Witnesses. That's pretty off topic, and crossing perilously toward religious flamewar. On the other hand, it was weird enough not to count as generic—except where it swerved into atheism vs. religion stuff at the end.
What we're trying to avoid is getting sucked into the black holes of the hottest controversies that online forums can't seem to resist but that people never have anything new to say about. That's the purpose of the rule. On HN, the organizing principle is intellectual curiosity, and there's no curiosity in repetition.