I'm not going to ascribe any moral grading to these NK soldiers, for all I know they were forced to deploy against their will, or maybe they volunteered.
Either way, it doesn't matter. They were active participants in a conflict, and deployed within a war zone. That means they were legitimate military targets to be killed, or captured.
You just described a genocide, like one of the plain text definitions of a genocide. Mass murdering a group of people to steal their land.
Doesn't matter that you've made up a hypothetical fantasy scenario where they aren't caged in cornered, but have free movement, it's still a genocide.
Oh, and they aren't free to leave, they are caged and cornered.
So you're wrong, and you're wrong again.