It's one thing for a dev to copy another dev, but to copy your own customer's success? I couldn't personally go that route, but they can and will obviously do whatever they want.
In the end, they can simply say their Fortnite customers demanded a Battle Royale mode and so they provided, simple as that I guess.
Edit: Imagine the scenario where you're designing a game with the UE and you're contacting Epic, sharing your ideas, because you need their technical expertise to get the engine to do what you need it to, or you simply encounter bugs in the engine that inevitably lead to some sharing of ideas or concepts during the reporting process, and down the road, there's your game mode in one of their titles. I'm not suggesting that's the case here, but that has to be in the back of the minds of anyone considering using it from here on out. It'd be like a swift kick to the chest that sends you through plate glass.