I read here it's an option which must be turned on, but maybe it's changed since then.
I hope someone from Google comes here and respond to this, but I believe all feeds have the "discovery" part in them, which means they all point to the google hosted hub, however, the realtime approach only works if the publisher pings the hub as well when its feed(s) have been updated, and this can be done with Pingshot.
This is the behavior we have seen at Superfeedr for these feeds.