Typically, when the ATV, or any streaming device, is connected to the TV via HDMI-CEC and the TV is the main controlling device or hub, then the TV will control the audio system which is also connected to it via HDMI-CEC. This allows the ATV to control the volume through the TV whether it is the TV speakers or a separate audio system. In a setup like that, then both the ATV remote and the TV remote can control the volume. However, when you hookup HomePods directly to the ATV, I don’t think the TV can control their volume, or even see them as far as that goes. The TV needs to be directly connected to the HomePods (or other audio system), but Apple does not allow that. Instead the HomePods can only be connected to the ATV via wifi if you want Atmos sound. Apple didn’t give the HomePods an HDMI-CEC port.
I don’t have any HomePods connected to my ATV, but I have an Echo Studio connected to my Fire TV Cube, and it is the same situation. Amazon didn’t give the Echo Studio an HDMI-CEC port. Instead it must be connected to the Fire TV Cube via Wi-Fi to get UltraHD or Atmos sound. So my TV remote cannot control the Echo Studio volume.