That's what I thought it sounded like too, which made me think of the strange part - as far as I know, any Android app provider can make their video app Chromec

That's what I thought it sounded like too, which made me think of the strange part - as far as I know, any Android app provider can make their video app Chromecast-compatible. It's up to the app provider, not Google. Amazon already has a Amazon Video Android app, which last I checked, they only provide over their proprietary app store. So it's up to them to make Prime Video work on Chromecast, they just have to put their Video app on the Play store, and set it up to Cast.