Reddit does have global rules about deceptive content manipulation (e.g. voting rings, bot farms etc.)

If this guy had disclosed his conflict of interest, he would just have been an obsessed crank and even as a moderator, that's his right. But when he didn't, I'd say it was large-scale manipulation, and it's clearly in Reddit's interest to not allow this sort of thing (especially now that they're selling all their data to AI companies).