There’s a misunderstanding here.
Swift the programming language is open source.
It doesn’t mean that iOS or OS X frameworks, like Cocoa, UIKit, etc, are available in Linux – Apple didn’t announce that.
When you make an app for iOS using Swift, you have to import UIKit and other libraries – those are the iOS parts.
Swift is just the language – you can already make an iOS app with Ruby, Objective-C, JavaScript, etc, using these same libraries.