The problem, though, is that "off-topic" (as judged by the guidelines page) stuff can get voted up. My argument is that if something gets voted up here (legitimately) then it's de facto on-topic, but that other poster disagrees and is wondering how to give feedback on that without breaking the rules.

Why wouldn't they get any comments or upvotes? You just posted a thread about a big spike in new users. There's no guarantee that they have read and taken to heart the guidelines.

I mean that if a submission actually is offtopic, users will flag it and editors will kill it. And once it's dead it will get no comments or upvotes.