You quoted the answer to that already. If these change notices are "blocking", then the sending device won't re-send the message until the user has verified it.

You quoted the answer to that already. If these change notices are "blocking", then the sending device won't re-send the message until the user has verified it. If the user hasn't enabled the notification, then the sending device will re-send immediately. This makes it trivial for the server to figure out who's actually enabled the notifications and who hasn't, which means the server can be confident about when it's actually safe to MITM.