Having immigration policy and not allowing immigration at all is not the same thing. Even Japan, whose immigration policies are very strict, has both sizeable number of foreign workers and immigrants too. US currently has very open immigration policy (1M immigrating every year) and a problem enforcing the immigration laws it has (tens of millions of illegal immigrants). It's a long distance between this and "no immigration" and there are many points on this way which one could choose to stay in.

Yes, but the reverse is also true. The soviet union, for example, allowed scientists to attend conferences in the west. It also allowed families to visit the ocean. So they did let some people out sometimes. Just as you say that Japan lets some people in sometimes. The end result is the same. Most people will not be allowed to immigrate or even travel in a nationalistic world. Whether it is because countries don't let people out or that they don't let people in is of no consequence.

A good example of this is the case of North Korea and China. NK has a very heavily guarded border between SK and NK. Basically no one has ever escaped via that border. However, people do escape to China, and China deports them back to NK. Is there a difference between NK not letting people go to SK and China deporting people back to NK? The end result, is the same. No one can leave NK.

> The soviet union, for example, allowed scientists to attend conferences in the west

Some scientists, for some conferences. It wasn't easy to get these permissions, and if you have security clearance, no chance. Maybe if you're word-class celebrity, not otherwise.

> So they did let some people out sometimes.

Yeah. It required to pass a lot of checks and having some connections. And depended a lot on where you're going - Bulgaria or Mongolia is one thing, Hungary or DDR is another, France or Spain yet another, and UK or US is completely different. Depending on how ideologically far the target country is, you'd require to be in the more trusted circles and have more connections to be allowed to get there. It's not like you just wanted to go there, you buy a ticket and go, nowhere near it.

> Just as you say that Japan lets some people in sometimes.

Not even close. You obviously know very little of both to compare. You can visit Japan as a tourist anytime. If you want to stay and work, it's harder, if you want to become a citizen, even harder. But entirely not comparable with USSR exit restrictions, not even close.

> However, people do escape to China,

Ah, you misunderstand the situation quite a bit here. People don't just "escape" to China. NK has a huge underground economy which is completely dependent on China. That's how NK has been staying afloat for decades now (well, that and international humanitarian help, which NK openly extorts by nuclear threats), otherwise they'd die out (well, not all probably, China would take over before that, but they don't really want this poisonous radioactive dumpster fire as part of their country or their official responsibility). Of course, this can not be done openly, so it is all done on pretend basis - Chinese and NK powers pretend it doesn't exist and from time to time deport some people for show. But they all know it's a pretense.

> Is there a difference between NK not letting people go to SK and China deporting people back to NK?

Of course there is. Ultimately, the welfare of NK people is the responsibility (and continuing criminal failure) of the NK government. China is not responsible for them, but in fact is trying to manage the situation while keeping the pretense of situation not existing at all, because it's their fault NK exists in the first place.