Sadly they don't specify why that's not necessary with iOS, I'm so curious how that's any different. They need some ID to send push notifications, and Apple keeps those registered to devices for delivery. I don't get how that's any different from Firebase push notifications and wish they could actually explain that.
Android does not require this if the app developer instructs users to disable battery optimizations for the app so that it can poll for notifications itself.
They say they'd have to maintain a DB of device info and user accounts to send push notifications, whereas Apple devices do not require this.