I understand the point you’re making, but all of my Apple TV’s are hooked into the high quality speakers, because that’s how the audio from those TVs is played.

And how should HomeKit know that?

What HomeKit sees is a video device. It could play audio, just like you could play video on your Bluetooth headphones. But, would that give the result you'd expect?

(You ask 'Hey Siri, play "A Good Day to Die Hard"!" and since your Bluetooth headpones are the device nearest you, it plays the video on your headphones. Audio only.)

If you ask HomeKit to play audio in a zone, it will play audio on the audio-devices in that zone. If you ask HomeKit to play video in a zone, it will play video on the video-devices in that zone.

(I moved the screen in my Lab to the Living Room zone, just to confirm that.)