Hi!
I did some digging around.
In the article "Play audio in Dolby Atmos or surround sound on your Apple TV" there is this smal text:
"*Some sound bars and AV receivers support Dolby Atmos, but don’t support video formats like HDR10 and Dolby Vision. In this case, you can plug your Apple TV 4K into either the sound bar or AV receiver to play sound in Dolby Atmos. Or you can plug your Apple TV 4K into your television to watch video in HDR and Dolby Vision. To do both, you need a television or an AV receiver or sound bar that supports both Dolby Atmos and high-quality video formats. Apple TV 4K uses a high bandwidth form of Dolby Atmos that doesn’t work over ARC connections."
So basically you need to plug in your Apple TV 4K in to your soundbar / AV receiver or use a connection that supports a higher bandwidth.
Now comes the bad part.
Because the Sonos Arc only have one HDMI port and, as you said, the only connection you can have is from Apple TV 4K to the TV and then from the TV to the Sonos Arc means your TV need to have eARC to be able to support the higher bandwidth and send it though to the Sonos Arc.
I understand that this isn't something you like to hear and I feel the same as I'm in the same situation with my Philips TV but I guess we can always leave a feedback for Apple.
The TLDR-version:
To use the Sonos Arc with your Apple TV 4K your TV need to have a eARC connection support.
