I vouched for (undeaded) your comment, since I think the flagging/downvoting runs contrary to the principles of discourse we're (at least implicitly) celebrating here, smelling of tribalism/favoritism. Just because you view someone favorably, and someone else claims something negative about that person, doesn't mean you just get to flagbomb them into oblivion.
The right response here is simply to ask for further substantiation, as danso did. It's not clear to me that the mods are directly responsible for what you've experienced. If you haven't, I would contact them for clarification.
Downvoting something for complaining about votes is fairly normal. Downvote baiting on top is flagworthy. I imagine lots of people quite sensibly apply these as a matter of course.
With some exceptions I downvote any comment that complains about downvotes. I'm not 100% comfortable about it, and I might change my approach, but so far rigorously applying the rules seems to be working. I'm willing to bet I'm not the only one to do so.
Suppose he is being treated unfairly by the mods. Is that not worth complaining about?
I don't give a crap about some silly "downvote every comment complaining about downvotes" rule. Complaining about downvotes was frowned upon because people get inexplicably downvoted as a matter of course here, and the frequency of complaints was deemed sufficiently detracting to warrant being frowned upon. Being treated unfairly by the mods, however, is another matter entirely.
I haven't seen a preponderance of evidence suggesting the complaint was ill-founded, and the guy's tenure and comment history (what I looked through of it) do not suggest he's an ill-mannered troll, so I reserve judgement as to whether the complaint is in the wrong. Like any reasonable person would.
The right response here is simply to ask for further substantiation, as danso did. It's not clear to me that the mods are directly responsible for what you've experienced. If you haven't, I would contact them for clarification.