This isn't true at all. Since iOS launched there has been UIKit and then this SwiftUI.
They didn't say anything about iOS. There have been lots of UI frameworks from Apple, some abandoned before they were even finished:
QuickDraw, Quickdraw GX, HIToolbox, AppKit, Cocoa Touch/UIKit, Playgrounds/IB, SwiftUI
The constantly changing frameworks and languages are a useful strategy from platform vendors - they do help provide platform lockin, and make it very very difficult to provide cross platform apps. That is a real threat to Apple with offerings like flutter from Google. Of course there are other reasons to make a clean break with the past, but there is a lot of work just to stand still in developing for a platform like Mac OS or IOS - they don't highly value backwards compatibility and often introduce sweeping changes. Constant change is in the platform vendor's financial interest, and I don't think it's unfair to point that out.
Just to take one example - apps built for the original iOS are now almost entirely obsolete, and it's not worth reusing any of their code in a modern swift app.