Not any more, the opensource vesion is here, runs on ubuntu and fedora (i hae got it compiled on fedora). Enterprising souls are right now working on a windows version.
In practice, nobody is seriously writing Swift code (or any other code) on iOS devices. My meaning was I don't understand why this is an iOS app instead of a Mac app or even a web one (like IBM's Swift sandbox[1]).
That being said, I'm kind of impressed someone managed to get an app on the App Store that can do this. I know that Apple loosened its restrictions on code execution on iOS a while back, but as far as I am aware there's no way to compile Swift on ARM… did they write their own compiler/runtime or something?
The lessons and exercises are limited in scope enough that they are entirely static. The work the app needs to do is limited to parsing, stylizing, checking (simple) syntax and matching user input to the task, not executing it. The lessons themselves are no more complex than a programming PPT presentation that looks prettier.