I've been playing Fortnite for almost 10 years, so I'm very aware of how the game goes. I agree Battle Royale is the centerpiece, and I'd expect HHN to take a lot from it. My point is that there's more to Fortnitemares than just Battle Royale. Over the years, it's also included Horde Rush, horror crossovers, Halloween quests, cosmetics, and the broader Halloween aesthetic.
So I don't think Universal using the Fortnitemares name automatically means they're adapting Battle Royale specifically. I think they're adapting Fortnite's Halloween identity, with Battle Royale naturally being a big part of that.