It looks like they took an approach similar to GNUStep, with its inverted layers compared to Apple's implementation. CF on top of Foundation on top of UIKit.

Looking a little deeper, it's missing some things... KVC/KVO are entirely absent. libdispatch looks to be pretty broken. NSNull is wrong in about 3 ways. There are several classes which are just stubs. Most of the NSLock subclasses are missing.

It's a good start, but I don't see it becoming the ObjC standard library by any stretch of the imagination.