Not sure the answer is what you want to hear.
No, the Sonos Amp won’t ’accept and distribute’ an Atmos signal. It can act, when ‘bonded’ to your Arc Ultra, as a driver for surround speakers.
Nor does the current implementation by Sonos of AirPlay 2 support anything beyond stereo, although that may (we hope) change in the future.
As things stand right now, your best bet is to pull the audio off the HDMI signal using an eARC generating device. There is the HD Fury Arcana, the Feintech VAX04101k, and the OREI HDA-935. You can also order from those links from the companies directly, or check your local retailers for availability. Amazon may carry some, depending on location. At this point, there may be more manufacturers, I haven’t been keeping track. You could then connect your Arc Ultra to this, and get the Atmos signal you want, then it’s just a matter of getting the correct speakers to connect.