Android TV allows customization for carriers/Cable STB makers. They usually haven't allowed this for retail STB like Mi Box and Shield. But either that's changing, or Google just said 'Fuck it, it's ours, we can do what we want'.

I think 'Google TV' on the new Chromecast is just Google's customized/branded version of Google TV - just like on the Pixel line - it's now Google's customized/branded version of Android - and not just stock Android anymore like it used to be (even though still pretty close).

The similarities in how the home screen looks on the Google TV, and how it looks on carrier's/cable co's customized Android TV STB units, is what led me to believe this.