Even if they sell the game on Amazon, they still pay Microsoft a royalty for each game sold. Whether you want to call it a "licensing fee" or a "Microsoft tax" seems like a matter of semantics. Also keep in mind that their publisher agreement with Microsoft gives Microsoft approval rights over the game itself, marketing materials, and even packaging. And if they sell virtual goods in the Xbox version of the game they have to go through the Xbox store and pay Microsoft 30% of that.
The amount of control console manufacturers exert over the games allowed on their platform is not dissimilar to the Apple and its App Store.
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.