The .NET stack inside UWP is .NET Core (for the most part), but the UI stack of UWP is WinRT controls [you can read that as modern COM controls] implemented mostly in C/C++ and directly a part of Windows.
So .NET Core doesn't benefit from UWP UI library, but UWP typically benefits from .NET Core upgrades.
If keepass were a .net standard library, you could have different front-ends for various platforms. Making the front-end crossplatform is a good goal too. I'm sure you could use bindings to Qt or gtk. There's various native crossplatform front-ends in the works too. See my comments below.