Yes you're right, Xamarin Forms seems to be targeted at Windows/iOS/Android portability with a common XAML codebase. For cross-platform desktop apps the best choice seems to be something like [Gtk#](
http://www.mono-project.com/docs/gui/gtksharp/] But I think the same principles would apply: the UI projects would utilize the non-platform specific code in AlephNote.App and the plugin framework etc. It would probably be necessary to have a separate Mobile UI project anyway, but it still would reuse a lot of the existing C# code.
I'm totally a beginner with Xamarin and Gtk# but I'd be willing to give this a shot. I really like how multiple backends are supported via plugins in AlephNote.