There’s a few comments like this in this thread, from people that I guess didn’t actually read the post :)
They weren’t asking how to do it; they were asking why it works out-of-the-box with the standard Home Assistant notifications.
You don’t need ntfy; the standard Home Assistant app notifications work anywhere since they route via Google Firebase.
I don’t see anything in that article that says that Google store the contents of the notification. It just says that they link push tokens to emails, which is true - they have to know who to send the push notification to.
In any case, if you don’t want Home Assistant notifications being relayed through Google, you can use a persistent connection so that the app connects directly to your Home Assistant server.