Power turns my lg tv on and off which is prob done through cec. I use my lg tv as my volume output to my receiver using optical, and whatever hdmi input I choose on the tv, the tv outputs the sound from that hdmi to the receiver through optical. I think there is only one hdmi connection on my receiver, which is an output to the tv for DVDs, but maybe I'll tinker and see if there is other options. But I doubt my receiver is cec, cause it's a nice sound system but only $200, you can get decent sound for cheap these days.
The question I think here is is it a Siri remote issue or an atv issue. A thought I had is maybe a lot of Siri remotes have faulty IR sensors. Or maybe the atv says its learned the commands but isn't sending them to the remote, software issue. I notice when I hit the volume buttons, the led on the atv blinks but that could simply be the remote over Bluetooth making it blink, not like it's receiving actual ir commands from the remote to the atv sensor.
What's so odd to me is on every previous atv, it could learn any remote and would work flawlessly. I'm close to saying this could be one of apples biggest debut flops, among the other issues with no remote app, the severe limitations with Siri, no Bluetooth keyboard support and beta App Store.