It's just that a mobile app that runs neither on Android nor iOS (90%+ of all phones) is not exactly universal.
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...
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...