Apple rolled out a low-latency audio feature a few weeks ago -- if you have a HomePod/pair of HomePods set as your primary output on your ATV4K, they're able to stay in sync with the TV even when you're doing something where any audio latency would be noticeable. For example, if I'm streaming a game from my PC, video and audio are in lockstep (which you wouldn't expect to see normally, since the audio would get 1-2s of buffer from the video) and there's no noticeable input latency (which you'd see if the device was holding back the video to build an audio buffer).