The walled garden absolutely prevents fragmentation.
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.
Ah OK, I see that point now. But Android's fragmentation isn't due to the users being unwilling to update, to keeps their apps working. It's because it used to cost a lot to OEMs and carriers to port their drivers etc to the new versions.
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.