Good point. I don't know how the HN software is written but my guess is that when it's time to take action (X number of flags), it's something a looks at.
There could be a level of automation built into this where X number of high reputation members flag something, it could automatically be banned.
Either way, I think flagging is more fitting to abuse than off-topic. I agree with you that it would be great to get a mod to weigh in on this.