But in that case they would eventually be caught red-handed and won't get to do the "for the children" spiel and get it swept under the rug like it's about to be.
Read evidence from past trials it is obvious. See also successful and failed attempts to subpoena this info from VPN services.
Only people with iCloud will be using the relay.
It is true on the surface the photos is disconnected from the use. However, Apple only needs a solid answer that handles the bad optics of what you can do with the Tor-like anonymity of iCloud Privacy Relay.
However, if you look more closely, the CSAM service and its implementation are crafted exactly around the introduction of the relay.