I'm not so sure but i don't think that could be the case tbh. Despite being seclusive country, they do allow student to study abroad and they allow their people to work abroad as well. Under what condition, i don't know, but the number is very limited and they also allowed back in for "reeducation". This article has a tiny bit of info on this. I suspect they made use of social alienation so even when they went back and tell others what's it like, no one will believe it because of how deep the indoctrination run.
Edit: to add, these are the top soldiers they sending, NK will definitely want them back.
Well, it's not like they have free will. All their knowledge of outside world and within country is filtered through their government, bringing back furry porn means nothing if they cannot share it. It'll be confiscated immediately on arrival.
Recently there's a lot of research about weight lost, and some of them found exercising isn't a great way to lose weight, because when you do it regularly your body learn to adapt to the energy required for that activity. They found controlling what you eat is the better way, but i do know some people can't lose weight that way either, no matter how hard they try. So this solution could be specifically aimed for that. Still, exercise is good for health though, pretty sure these jab will not have the benefits from exercising, and someone could very well rebound if they eat the same way that make them gain weight.
Also i don't think the accessibility of gym is the key, exercise cost nothing if you go out and jog or ride a bike.
永固城 pinyin is not Ba Sing Se, it's Yong Gu Cheng. I genuinely have no idea where that coming from.