That's an interesting question.
Couldn't they distribute a "Fortnite loader" through Play store? Since Fortnite is free-to-play, the loader wouldn't charge anything, so it wouldn't technically violate any policy. Seems like an interesting workaround to avoid giving Google the paycut.
But what if you distribute "Fortnite"? Free to play, no premium functionality, free "upgrade" - except that the upgrade is from outside the app store. And once you get it, you can start paying for stuff. I don't think this qualifies as "an app store", since it's not an app store/ you can't download other applications from it (just the premium version of this one).