>> The obvious conclusion is Apple will start to scan photos kept on device, even where iCloud is not used.
Wrong [1]. It's even in the first line of the document which you apparently didn't even read:
CSAM Detection enables Apple to accurately identify and report iCloud users who store
known Child Sexual Abuse Material (CSAM) in their iCloud Photos accounts
1. https://www.apple.com/child-safety/pdf/CSAM_Detection_Techni...