>The system is essentially scanning for the signature for some known set of images of abuse

Huh? The system is scanning for landmarks, not images of abuse.

>people will be caught this way

Due to the homomorphic encryption, I don't think Apple even knows whether the image matches a landmark in Apple's server database or not. So even if Apple put some images of abuse into its server database (which Apple claims only contains pictures of landmarks), I don't think Apple would know whether there was a match or not.