According to this article you can play Dolby Atmos through a compatible Soundbar then it says the only audio format choices are stereo and "Dolby Digital 5.1". Do the consider Atmos the same as Dolby Digital 5.1? I can't get it to play Atmos.

• You should set the Audio Output to ‘TV speakers’, then set the TV to passthrough audio to the soundbar. If you select the soundbar as output in Apple TV, then it will use AirPlay or Bluetooth (both not HDMI) and in stereo.
• Dolby Atmos from Apple TV with passthrough audio through the TV, requires that the TV supports DA too, and that the TV has eARC. Regular ARC doesn’t have enough bandwidth for Dolby Atmos in Dolby MAT (uncompressed) format that Apple TV uses.
• Audio systems with video passthrough will avoid the eARC issue, but Sonos doesn’t work that way.
• Without using TV audio passthrough eARC, there is an option to route around that with an additional HDMI processor-adapter in the HDMI chain, e.g. using Arcana.

Set up Dolby Atmos or surround sound on Apple TV with a sound bar or AV receiver - Apple Support

Play audio in Dolby Atmos or surround sound on your Apple TV - Apple Support