Just a correction for you, there's not a list of approved images. The CSAM database are a list of illegal (unapproved if you will) images.

Other than that, yes it's possible to add noise to an image so a perceptual algorithm misidentifies it. I described the false positive case but it can also be used for false negatives. Someone can apply noise to a legit CSAM image (in the NCMEC database) so Apple's system fails to identify it.

The false positive case is scary because if it happens to you your life is ruined. The false negative case just means people have CSAM and don't get found by the system. I'm much more concerned about the false positive case.

Keep in mind that there are multiple straight paths from the false negative case to the false positive case. I'll give you one examples: pedos can and will use the collider to produce large batches of CSAM that collide with perfectly legitimate images (e.g. common iPhone wallpapers). They literally have nothing to lose by doing this.

Eventually, these photos will make their way into the NCMEC database, and produce a large number of false positives. This will also make the other attacks discussed here easier to execute (e.g. by lowering the human review threshold, since everybody will start with a few strikes).