Apple's site is a bit contradictory on this. In another place it lists speaker features and it lists "Home theater with Apple TV 4K" as a feature of the full size HomePod but not the mini.
Home Theater uses the full size HomePod's to do 5.1/7.1/Dolby Atmos surround sound. It requires the spatial processing to allow for the surround sound experience.
The HomePod Mini can be used as a stereo pair, and connected to an Apple TV with no issues.
Inquiring minds want to know.