I'm guessing here, but wouldn't they just push the e2ee message through APNS? Then decrypt client side. Or does Apple require plaintext messages for push notifications (that seems bad if they do)?
> [...] never include sensitive data or data that can be retrieved by other means in your payload. Instead, use notifications to alert the user to new information or as a signal that your app has data waiting for it.
https://developer.apple.com/library/archive/documentation/Ne...
https://developer.apple.com/documentation/usernotifications/...