For what it's worth, I'd already voted to close it as off-topic, and although two respected moderators have said otherwise, I secretly continue to think it's off-topic, pretty much for the reasons you mentioned.
But I also think that's not a good reason to flag it. There's already a mechanism in place for regular off-topic questions; you vote-to-close, it enters the review queue, others consider the issue, and a community consensus evolves over time. Flagging for moderator-closure-as-off-topic is something I've hardly ever done, and would only do for egregiously off-topic posts.
I think it's laudable that you want to get involved in maintaining the quality of the corpus of SF questions. Good for you! But I also think the right way for you to do that is to continue to write good answers, collect upvotes, and get the rep to cast close votes - then put 30 minutes a day aside for doing your review queues, like many of the rest of us.
As has been made clear to me on several occasions, moderators are there as exception handlers, not as a substitute for community consensus. The question you quote doesn't, to me, qualify as needing an exception handler - it just needs three more close votes - or not!