They must live in quite a bubble if "universal" only includes Microsoft platforms.
I guess Google and Apple also live in a bubble? Not really sure what your point is. Is your expectation that Microsoft Universal apps also run on Android and Google? On Web browsers? Would you like them to use a different term?
If all it does is run on all Microsoft platforms, from Windows Phone to XBoxOne, it's just a "Microsoft" app. If Microsoft decides to call what the Xbox runs "Windows" they'll become... Windows apps...