Ok, confirmed. You don´t even need the homepods set up as the default speakers for the apple tv for the home theatre processing to work with 5.1,7.1 and Dolby Atmos tracks.
Final, updated guide, with the help of apple support:
A). Update the Apple TV and the Homepods to the final public released version of 14.2 (betas wont work).
B) Make sure that the Apple TV and the homepods are in the same room in the home app. While you are there, go to the Homepods settings in the Home app and make sure that the "Sound Check" option is off (this affects the dynamic range and volume of everything played through the homepods, and is not recommended)
C). In the Apple TV, go to settings Audio/Video menu and make sure that the "change format" option is off, and that the audio format is set to "Auto". While you are there, turn off "reduce loud sounds" as well (this affects the dynamic range of everything played through the apple tv, and is not recommended).
D). Select the Homepods from the control center of the Apple TV. Then go, for example, to Netflix, and start playing Hubbie Halloween or the Irishman (again, for example). While playing, swap down on the Siri remote to bring up the audio and subtitles menu . You should see the Dolby Atmos logo there (in the info tab), which indicates that your are indeed playing a dolby atmos track.
Same with Apple TV+ and Itunes purchases.
Unfortunately, the Atmos logo doesn´t appear on Disney+ streams, only a 5.1 badge, which tells me that the app need to be updated to work with the Homepods . However, the Home Theatre processing of plain old 5.1 tracks by the Homepod is also very impressive.
Turn up your Homepods volume and enjoy!