I've worked on many iOS and Mac apps. I'm honestly not sure how anyone can claim AppKit and UIKit are essentially the same.
The frameworks are not identical, but to say they're not related… they only look wildly different if you're focusing in incredibly closely. If you consider .Net, or Java, or Rails, or GTK, or almost anything else, the differences between AppKit and UIKit are miniscule.
Your examples of delegates, responders, targets etc are concepts - and indeed UIKit and AppKit are both built around the same concepts.
The implementations of both is quite different however, to the point that porting something of complexity from UIKit to AppKit involves a lot of tedious changes - or worse: creation of abstraction layers if the codebase is to be maintained for both apps.