Earlier quoted context omitted.
Depends on what you define as their game store. I can purchase a game that is playable on the xbox from anywhere that sells a copy. One of these places is the digital store that comes with the xbox. But I can also go to cd-keys, or amazon and buy a copy there that will still work on an xbox.
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.