Homepod mini definitely plays dolby atmos when playing music from Apple Music.
i don't know about movies since my minis are in my home office and there's no apple tv there to test out
but for music i can definitely hear it. Even if it's not indicated in the music app. Maybe it's a bug but i tell you it works
i can instantly differentiate between lossless and dolby atmos. I also have a pair of Homepod Og's and can clearly hear the difference between lossless and dolby. That's playing music directly thru the minis no Apple tv involve. both on 15.1