I thought it would be a lot more useful than it is.
It's almost like Apple Music without the app. (Does that make it ple Music? Ha.) You start music playing with your voice, but in order to control it you have to either use the Google app or the crummy controls on the Nest itself. So you're exactly right: the Apple Music app is not aware music is playing.
It's "old school" but I find Bluetooth more useful in this regard, so I can just play/pause/etc. it directly with the native music controls. I don't want to have to shout across the room just to turn down music or skip a track.