Correct, that's currently the case. Users can use QR code backup/restore functionality if enabled by the developer to switch between iOS and Android. That would have to be done app-by-app. We're working on our own cloud backup system to automate this.

I think such transitions between smartphone OSs already entail significant credential transfer issues, since saved passwords also do not automatically move between OSs.

You'd have similar problems if you used "Sign in with Apple" for an app on an iOS device and then switched to Android.