They must live in quite a bubble if "universal" only includes Microsoft platforms.
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...
I doubt its a very high priority though given how small the OSX Desktop market is in comparison.. it'll just be a very slow merging ;)
Sadly the other platforms died so the portable executable is really just the stuck executable.