Right, it was introduced publicly as an API in Mac OS X Leopard, but my understanding has always been that it was actually conceived as part of iPhoneOS, then ported to Mac as well. The iPhone shipped a few months before Leopard, though of course UIKit wasn’t made public until the next year when the iPhone SDK was first released.

There’s a similar story around SwiftUI. It was initially created by the Apple Watch team at Apple, then extended to all the other OSes (all this was long before it was made public).