> UXKit is just a simple compatibility shim on top of AppKit. There are many like it, and there's nothing particularly interesting about UXKit except that it is used by the team that developed the Photos app.

Yes, it is, but I think you're underestimating the value of a "compatibility shim". Sure, right now it's all backed by AppKit, but once developers are writing for this abstracted API it makes it possible for Apple to pull out the underlying AppKit implementation details and use a UIKit adaptation instead.