I know my ATV 4K allows me the option of using AirPlay for audio output through my AirPlay-capable receiver. Nobody will probably know for sure about what you're asking until AirPlay 2 finally launches. In any case, you can do a lot better than two HomePods for use as TV speakers for $700. I'd be worried about audio and video syncing up through AirPlay as well. Normally, the device AirPlaying is doing so to both the video and the audio, in what you're asking though, the video is being output through the HDMI, but audio is going over AirPlay, so each are taking a different path.