How do you suppose APNS knows which device to deliver the notification to?

Something that... links it to the device? Like, a unique ID that Apple can identify?

It sounds like he thinks HE has to store this information, which is simply incorrect. It will obviously be stored by Google in Firebase, and by Apple wherever that gets stored, but HE does not have to store it.

I write apps for a living. I have users subscribe and unsubscribe to channels, and at no point is there a user account with password involved in either iOS or Android. If you want the memory of which channels they have subscribed to to persist across uninstall/reinstalls or different devices, then yes, but for an app like this you don't need to persist those settings.

At any point the government could subpoena who's received pushes (or at least, who's registered to) from both Google and Apple.

You don't, there's privacy respecting ways of delivering notifications in android.

Also, a 24/7 connection to a server isn't nearly as bad as you might think.

The connection isn't active the whole time, it only uses any significant amount of battery if there's actually data being sent or received. You likely already have quite a few of them anyway, how do you think systems normally listen for push notifications?

Besides all that, I read in other comments that the privacy issue was the device id firebase needs. Obviously apple also needs some kind of device id, otherwise how do they know where the notifications are going?

Did some searching, yup apple also needs a unique identifier:

When it’s time to send a notification, you generate a request that contains the notification data and a unique identifier for the user’s device.

From https://developer.apple.com/documentation/usernotifications/setting-up-a-remote-notification-server