

The same way with iOS. At some point, the third-party service have a way to link a push to a device. It does not mean that you can link an user to a device, or a specific request to a device. You get a unique ID for the notifications, yeah. And someone could tell that the app server have these ID. But that’s not particularly different with iOS. It not being exposed to the app dev directly does not mean that this info does not exist on the third-party server, that can still get asked about it.
Unless Apple found a way to magically send a message to a specific device, from a specific external server, without anyone, anywhere, having any idea where the notification should go. Which, fair, could be done by sending every messages to everyone after encrypting it for a specific recipient, but that would be a bit inefficient at this scale. The trace for push notifications exists, whether you’re using Apple or Google as the backend.
I find it reassuring that some people are not proud of grabbing random people off the street to send them to their death with a smile.