The South Korean constitution treats North Korea as an integral part of its territory, currently occupied by hostile forces. Therefore, anyone from North Korea is automatically, and in fact always has been, a citizen of South Korea. It's not just asylum, nor "de facto" citizenship. It's full citizenship, period.
There's a mandatory program that every refugee must go through, not only to get them accustomed to South Korean culture but also to filter out spies and criminals. You will be under surveillance for a long time afterward. But even if you turn out to be a spy, you are still a citizen of South Korea and will be punished as such. The law simply does not recognize any such thing as "North Korean citizenship".
What's the legal basis for making their integration program mandatory if North Korean "refugees" are full citizens? What law would a person who refused the program be charged under?
>Who are the occupiers of North Korea if there are no North Koreans?
South Korean criminals I'd assume? It'd be like if some American militia suddenly seized some area of land and declared themselves a new independent country and began oppressing everyone already living there. The US wouldn't recognize that (and would move to stop them), but while of course the civilians there wouldn't lose citizenship, the militia wouldn't legally lose citizenship either. They'd just be criminals. Citizenship is a fairly big deal and it's not trivial to renounce it. In the US at least IIRC you literally cannot renounce citizenship domestically at all outside very rare exceptions, you must be abroad and do so at a consulate or embassy, and it's something considered not automatic and instant. Additionally one can be charged an exit tax depending on net worth and tax status.
SK of course will have its own rules, but I'm just saying as a matter of law "everyone in that area of our country is citizens of our country being illegally coerced/controlled by other citizens of country, who are criminals" wouldn't be that strange. Although often countries facing such a de facto split work something out legally, there's no inherent reason countries can't refuse to legally recognize things indefinitely.
> Who are the occupiers of North Korea if there are no North Koreans?
Rebels, traitors, and criminals (at least, those claiming to be the North Korean government, or its active adherents), just as was the case of the self-described Confederate States of America within the terriory of the USA.
There's a mandatory program that every refugee must go through, not only to get them accustomed to South Korean culture but also to filter out spies and criminals. You will be under surveillance for a long time afterward. But even if you turn out to be a spy, you are still a citizen of South Korea and will be punished as such. The law simply does not recognize any such thing as "North Korean citizenship".