You’re confusing two entirely different features. One feature does blind CSAM detection on iCloud photos. Any photos that are being added to iCloud in the USA

You’re confusing two entirely different features. One feature does blind CSAM detection on iCloud photos. Any photos that are being added to iCloud in the USA have a safety voucher calculated on-device and attached to the upload. The device doesn’t know if there’s a match or not. When the uploads reach Apple’s servers, if there are enough matches, Apple has enough information to decrypt the safety vouchers. The photos are then additionally compared with a second, secret hashing algorithm. If those also match, a human reviewer checks a low-res thumbnail to confirm. If they also confirm, it gets reported. So the initial calculation of the safety voucher is done on-device, but the matching can only be completed on the server, and nothing gets reported unless multiple other checks also match. The other feature uses machine learning to determine whether or not a photo attached to a message contains nudity. If such an image is received by a child’s iPhone, it’s blurred out and they are told that if they choose to view it, their parents will be told about it. The entirety of this scanning happens on-device.