Shouldn't there be an exception to that rule? "Unless the original discussion is about downvoting, or about an aspect of HackerNews that would lead to a comment about downvoting being on-topic". Of course, there still needs to be something of substance in the comment.

The comment we're talking about isn't quite substantial. Even the meta-discussion that's happening around most of this stuff isn't all that useful. There have been a lot of people who haven't really gotten the hang of commenting here, complaining about the power of a downvote.