UWP apps that work by accident on phones is not a bad strategy, though. Good application design is responsive, same reason responsive became a huge buzzword in web dev. Even if you are just designing with Desktop in mind: there are hundreds of combinations of monitor resolutions and DPI settings, and everyone has their own personal preferences for how they arrange applications on those monitors. Windows apps have always been resizable and there are always people that will want to shrink app windows to tiny phone sizes while they work on other things.
Just because UWP targets all Windows 10 platforms, it doesn't mean a developer will take all of them into account.
Currently most UWP developers only care about desktop as target platform, so if it happens to work and be a good UI/UX in a e.g. 7" display it will be by accident.
One needs to take responsive design and API contracts into account, to make a good UI/UX across all supported platforms.
Just like Google and Apple get phone apps that work by accident on tablets, but feel out of place otherwise.