HDMI 2.1 is arguably one of the most significant upgrades to the HDMI standard. Originally announced in November 2017, HDMI 2.1 significantly increases the maximum bandwidth capability of HDMI up to 48Gbps, and even more when compression is used.
www.rtings.com
The Apple TV uses/sends compressed sound. I'll just paste a portion of the above link because I will mess it up,
ENHANCED AUDIO RETURN CHANNEL (EARC)
The Audio Return Channel, or ARC, has been a part of the HDMI standard since version 1.4. ARC allows you to connect all of your devices (Blu-ray, game console, etc) directly to either your TV or to the receiver, and have a single HDMI cable connecting to the other device. This enables bi-directional communication between your receiver and TV; audio is sent from the TV to the receiver, or the receiver sends video back to the TV, with only one cable. This is great for a wall-mounted TV as it helps keep your setup clean.
The current implementation of ARC has some limitations, though; the low bandwidth (~1 Mbps) limits the quality of the audio that can be sent over the return channel, making it impossible to send high quality, uncompressed audio to your receiver. eARC solves these issues, as long as you have a compatible receiver as well. It vastly increases the amount of bandwidth that the ARC channel can use, up to an impressive 37 Mbps. This extra bandwidth makes it possible to send uncompressed 5.1 and 7.1 audio, like Dolby TrueHD and DTS-HD MA, which are often the carrier signals for Dolby Atmos and DTS:X.
From another article
The Apple TV can’t passthrough DTS-HD MA or TrueHD...
The compression method that the Apple TV uses is very good but if Apple is going to state 2.1 then it should be 2.1. Other than that I have nothing but luv for the unit. I hope, maybe the next version will be able to passthrough uncompressed audio.