Having a middleware to abstract firebases API from the services makes sense if just to minimize the disruption the next time Google changes it. I've always left tokens up to the callers to sort out before calling the notification service, but I see the appeal of centralizing it if you have a lot of small products. I agree there is no longer any good reason to directly target APNS.

Nothing in the article really said how they got to millions an hour. In my experience the notifications are cheap enough to send that that figure is easy to obtain without any special effort on the push side. It's also embarrassingly parallel/easy to scale. How everything else scales around that (recipient filtering, reporting) is the interesting part.