The signal path for me is Apple TV (Dolby Atmos on) -> LG GX (eARC output, PCM) -> Anthem MRX 540. I can see this is down mixed to stereo. I don't use native apps on the TV.
I was very interested in this device and looked into the usage.
I have a LG tv, you can convert Bitstream input to PCM in the TV, although any dolby content would be limited to 2-channel.
I also have an Apple TV 4K, if choose to turn off dolby atoms in the settings, the dolby content is decoded...