It will be interesting to see if Apple gives the same control over Apple Music and the updated iTunes that they provide with the Remote app.The moment Apple announces a multi-room wireless speaker system is the moment Sonos dies. So I'd say the team over at Sonos will be mightily relived that Apple has apparently abandoned any plans to kill them in the marketplace.
Currently, if you run iTunes on a Mac (not sure how it works with Windows), you can independently send audio from iTunes to any or all AirPlay devices use the Remote app. Having this capability with 30 million songs, without needing to use Airfoil, would be a great addition.
Also, iOS is capable of sending out a stream of audio to multiple AirPlay speakers using the Whaale app. Therefore, there isn't a technical reason that they can't implement multi-room audio without ever needing a computer in the mix.