Earlier quoted context omitted.
I think the point is that competition will prevent them from doing that. And it worked: in response to Epic's lower rates, Steam also cut their rates. If Epic raises the rate to 30% they'll be more expensive than Steam, and won't be able to compete.
I just checked and Fornite is not available on Steam. So all that will happen is that certain games will exist only in one store and not be subject to competition. Or they will secretly sell your data to third parties which Apple won't do.
You might argue that consumers are somehow injured by Fortnite existing on the Epic store but not on Steam. But I think that's pretty hard to prove, at least under existing antitrust law: Fortnite is free, and its microtransactions are not particularly more expensive than competitors, and if you don't want to play Fortnite because you don't like the Epic store for some reason, there are dozens of competing battle royale shooters on every platform Fortnite runs on (and, now, even platforms Fortnite doesn't run on). Where is the lack of competition? Or the injury?
Epic's case centers on injury not to consumers, but to developers. That case is easier to make, and it's the case they care about because they operate a game store that doesn't exist on iOS due to Apple's policies — and a game engine business that operates on a 5% revenue share model, which naturally would increase its profits if Apple stopped taking a 30% tithe from developer revenues in the first place. They're perfectly willing to sacrifice Fortnite Mobile, a comparatively small amount of revenue, temporarily if it gets their other business streams more income in the future.
