I don't know enough about Korean law to suggest whether or not this case has a credible chance. I honestly hope it doesn't.
I do know enough about the history of games to take issue with `PUBG is just an FFA FPS, nothing new at all here` comment.
Making the playable area shrink in size to make "last man standing" gameplay work with a reasonably large open world map is a brilliant feature. It's a real upgrade to other methods I've seen to add predictably to the length of match outcomes. Way better than just showing a hud icon to where everyone is after a certain length of time.
Counterstrike ruleset matches on battlefield sized levels are glacially awful without something like that. Every other game I've seen up til' these to attempt that formula does so with kind of a "meh." and it never ends up being the best mode.
The "Battle Royale" genre admits in its name that those mechanics were inspired from the movie "Battle Royale". Despite that, I can't think of any other game that applied this technique quite so well. Maybe there was a Tribes 2 game mode that experimented with something similar? So... do they deserve exclusivity? I don't think so..
For now though, I at least have to applaud PlayerUnknown for the innovation. Kinda sorry your good idea got turned into somebody else's money printer. :/
