For this just to be a "recompile in Visual Studio" deal, they'd need to provide a complete suite of iOS APIs, libraries/frameworks and hardware abstraction.
I very much doubt they've done that; mostly because they haven't even done it for their OWN platforms yet. Hell, even .NET Core is mono-based currently, and porting that to Linux and OS X is a fraction of the work needed to provide substitute libraries (or translation) for the entire of iOS.
Yes, you can build a Windows 10 Universal application now, but it's not like you can just take an existing Windows 8.x app and recompile that and have it magically operate properly under Windows 10 as a universal app.
Can't imagine this being more than being able to re-use objective-C code that doesn't use the platform's native libraries etc.
There is a relatively small startup called "App Portable" which has done this. They have converted the majority of Core Foundation to work for android. I think they have everything except Core Data.
So you can recompile iOS apps for Android platforms.
I see no reason why Microsoft with their almost infinite amounts of resources could not do the same.
Also remember Apple has made Core Foundation open source, so porting isn't impossible.
Edit: I also think that Swift will also work with this. Swift compiles to native code using the same OBJ-C Core libraries.