The restrictions for leaving and entering have not been imposed on them externally, this attitude of Korea predates even the Roman empire
Anyway, we’re at an impasse here. You’ve decided that the DPRK is not a distinct country and that all you need to know about their laws can be extrapolated from the ancient history of the Korean peninsula, and that anything modern which contradicts your juvenile interpretation of ancient history must simply be made up. I have no idea what species of brainworm is responsible for this ridiculous conspiracy theory, and I am not qualified to exterminate it.