They just repost kcna.kp stories, but the commentors are often taking the piss

No, I don’t think so, the tractors story is also on kcna.kp: http://kcna.kp/en/article/q/373160bb40729c78652eafd01caedc69.kcmsf

“You have been banned from r/Pyongyang” was a very popular meme response to comments criticizing DPRK fo a long while. Pretty sure whoever actually created that subreddit was actually riffing off that.