That's pretty weak, it relies on my phone needing to be a base for the operation, whereas Chromecast is decentralized, allowing the device to pull the feed directly from the internet itself.
Plus Google Home already ships with Chromecast, which has had multiroom support for like a year now.
They have airPlay, so I assume any app on your phone can cast to them (but you're going to lose some quality in that transaction I'm sure.)