If it’s the only way to make games for Xbox then it’ll be supported by both engines fully. If it’s some evolution of UWP and it’s a more inferior alternate way to develop then most likely it wouldn’t be supported by Unreal.
The thing is though, if you’ve got a good project, which you can execute, then you should be able to get into the Xbox dev program, it’d just be more expensive, and at this point it’d be a problem with the next generation console coming probably before you release.