Good News!
LG Support answered me today. There is an update (05.10.15). It cannot be download over-the-air (yet). But from their website: https://www.lg.com/de/support/product/lg-OLED77C9PLA
1. Download
2. unpack the zip file
3. Put the extracted file in a folder named "LG_DTV" on an usb stick.
4. insert the usb stick on the tv usb port
5. start tv
6. "update now?" -> kick "yes"
Now Dolby Atmos with 4k 60hz on my OLED C9 (from 2019) works fine with Apple TV 4k (2021 Model) and Denon Receiver just fine! nice!
Also, consider using an eArc setup where all your video sources go right into the TV and the TV sends the audio via one cable to your receiver. Both receiver and TV must support eArc. I did it and now LG can’t mess with me!