It is impossible to send a (edit: true) push notification to a device without knowing which device it is going to. The developer may not know/have access to that information, but Apple/Google know which devices they are sending those pushes to. If it wasn’t a true push notification, then they would not arrive in a timely manner and potentially only when the app was opened the next time.

He was using true push notifications, so the government could just subpoena that information.

He could maybe obfuscate who initiated the initial message, but its impossible to do that for the receivers.