Haxe is cross platform in the sense of being able to compile to different languages not in the sense of merging different platform features into one library.

For example, you can use Haxe to build a native Android application using the Java target as you would do with using Java in the first place. Same with iOS (but using the C++ target)

AFAIK, there isn't any active project that offer the same as Xamarin in Haxe. You could, in theory, use Haxe to target C# and Xamarin but I haven't tested it.