Both use Linux under the hood. You can even install LineageOS on some TVs.

The only reason AndroidTV is bullshit is the manufacturers because casual users want shit like Netflix and Prime preinstalled. Google TV in particular comes with a lot of crap and the ads, which believe it or not some users take as a feature.

But that’s not inherent to Android TV as an OS, it’s exactly like Android phones and manufacturers preloading a bunch of crap to make an extra buck. If your run AOSP you get none of that crap, and it’s fully open-source.