The one thing they explicitly do not have on the roadmap for .NET Core is any sort of desktop environment / UI stuff.
There won't be a Forms or WPF port.
There won't be a Forms or WPF port.
Xamarin.Mac provides C# bindings to Mac desktop APIs, and has nothing to do with Xamarin.Forms.
But it sounds like you are actually thinking of Xamarin.Forms, which allows sharing UI code on mobile platforms as well as UWP. You are correct that there is no Mac support for that.