That's ridiculous.
I don't recall id Software suing 3DRealms, Valve or any other devs when the first-person-shooter genre started to get 'copied' by the hundreds following the release of Doom (and even if it did actually happen, I'm 100% sure that no case was ever won), or like someone mentioned above, Blizzard suing other MMORPG devs out there for having copied gameplay mechanics (or just the genre itself) and trying to - lo and behold - making money off of the idea.
The bigger problem, however, is whenever someone (a development team) claims that a genre they supposedly invented actually belongs to them (because big news here: it doesn't).
This is - ultimately - a jealousy problem. The PUBG devs are simply pissed off, and are envying Epic Games because their version of the Battle Royale genre is simply better (and hey PUBG, here's an important hint: it's F2P). Well, tough luck PUBG. Look back in the past and see how many other devs out there "invented" a genre and then got royally dethroned by a better, more refined follow up. Just freakin' man up, refine your own game, make it better or make a sequel and set it up as F2P right from the start. But no... sure, suing Epic Games is easier and more importantly, makes more sense.
In fact, a similar (albeit not identical) scenario already happened before with id Software and Epic Games back in 1999. That same year (and within the same period, around the holidays) both Quake III Arena and Unreal Tournament came out. We know what happened next (sure, Q3A was good, but the UT franchise became the king of the hill... so to speak). However, none of them ever got in a war of words (except maybe the fans of one game versus the other) and no one tried to bring the other to court to determine which one of them actually "invented" the arena shooter genre and which one might have copied the other. Anyway, that's just one example out of literal hundreds in the gaming industry. Usually you'll see someone suing another related to some patent or a peripheral or actual game code or something along the lines; some of those cases are genuinely worth fighting for. But holy sweet bejeebus those PUBG guys seem to be extremely susceptible people.
Yeah, Fortnite is a better game than yours, fast! Let's go to court! ... wow. Sorry, this story pisses me off; I perceive them (the PUBG devs) like children, something along the lines of one kid desperately trying to convince the other than his dad is better.