MacRumors says that both generations of AppleTV 4K support Dolby Atmos.
MacRumors - Old Apple TV 4K vs. New Apple TV 4K (2nd Gen) Buyer's Guide
"The only difference in terms of video and audio capabilities between the two Apple TV 4K models is support for high framerate HDR."
Play audio in Dolby Atmos or surround sound on your Apple TV
How to update your Apple TV
What’s new in tvOS 14 for Apple TV
"On Apple TV 4K with the latest version of tvOS, you can play sound in Dolby Atmos"; the tech specs for Apple TV 4K list Atmos and the ones for Apple TV HD do not...