Wait, what? It doesn't matter where you buy an XBox game, Microsoft is getting their cut as platform holder no matter if digital or retail.
That is the relevant comparison here, if Epic want to provide Fortnite on iOS they HAVE to use the app store and associated requirements. If they want to provide it on XBox, they pay a licensing cost for the proprietary tech they leverage from Microsoft. But they can choose to sell the game on Amazon if they want.
The amount of control console manufacturers exert over the games allowed on their platform is not dissimilar to the Apple and its App Store.