Yes I've heard of it. No. It's the same issue as always.
It's not that C2 JS can't be ported to other languages.
It's that C2 JS relies on too much Browser specification API's. As long as C2 relies on the Browser APi, then C2 is depended on Browser. So for some one to make a Xamarin port, would require that person to port the Browser api's to Xamarin.
If C2 JS used it's own API layer, then a it would be easier to make shims to other platforms.