You respond to those two lines by saying that the walled garden prevents fragmentation.
It does not.
* If you were unable to run apps other than via the Google Play Store in Android phones, the OS versions would still be fragmented. App developers have nothing to do with that.
* If only Google manufactured and updated Android phones (hence no fragmentation), you would still be able to run whatever you wanted in the phone. "Walled garden" doesn't mean "closed source".
Apple has on multiple occasions over the years removed apps from the stores that weren't updated to use the latest iOS SDK. This meant that since all apps are targeting the latest iOS there is little impediment to moving the entire platform forward.