Earlier quoted context omitted.
I would expect this just to be basic Swift support including -perhaps- the Foundation library (String, Array and other "low level" functionality). I don't think this Swift port has currently any easy way to create apps with a graphical user interface using some sort of Java bridge. So compared to Xamarin, using Swift for Android apps would be pretty limited. But please correct me if I'm wrong.
Swift String, Array, etc. are not part of Foundation, they are part of the swift runtime library. They have mechanisms for converting to and from NSArray/NSString representations, though (and because of that, their internal structure is quite similar). But that mechanism is disabled in Linux (and obviously I assume Android).
NSString/NSArray and friends (along with the bridging code to go between them and the Swift stdlib types) are part of the open-source Foundation, though. Eventually, that should be present in any reasonably complete Swift runtime environment-- it includes APIs, like networking and date/time handling, that would typically be considered essential but aren't part of Swift's stdlib.