This is the post I found btw from Tim on Reddit, indicating how they don’t stop the end-user implementing UWP themselves (last sentence): https://www.reddit.com/r/unrealengine/comments/4bc1dc/uwa_ue4/d183lk7
I think that explains it better than I could really. There’s UWP sample stuff on GitHub too: GitHub - microsoft/Windows-universal-samples: API samples for the Universal Windows Platform.
As I say, if you have the chops you can still support UWP if you really want to.