It's referring to React Native and Xamarin, whose Windows implementations are built on top of UWP XAML/WinUI. Xamarin only supports Android and iOS besides Windows. For React Native, there's additionally a Mac OSX implementation that Microsoft has been developing. At one point Canonical was working on a desktop Linux implementation but it seems to have been abandoned from what I can tell.

No it means you may be able to get it working on more than one windows PC. Don't bet on it though.