This doesn't have anything to do with the Chromecast. You still have to have a secondary device hooked up to the TV. Yeah they copied the basic principle, but until they sell something for $35 that lets you do it, they don't really have anything.
That this update is only available for the latest tablet is a very good reason to avoid buying Amazon-branded devices. The first HD has a camera and microphone - features touted as being new on the HDX, they just don't have proper software support because Amazon has completely failed to stay current with the Android trunk they're based on. There's no reason to believe that Amazon will do any better with the HDX. Rather, this large update comes now because Amazon's dev teams couldn't make the deadlines incorporating all the planned features.