Again, I don’t see how that is different from android.

For android, you need to buy into all the ecosystem of Google to access their push notification service.

You can use android without google’s system, but you can’t use google push system.

Because on android you can install another background service that can replace the features of the google service. This is pretty much what the Chinese did when they forked Android. There are alternative app delivery/update mechanisms that make this point entirely moot.

On iOS you just can't unless you are fully blessed by Apple, and this is precisely the issue.