This is a great answer but that’s not actually the GP’s contention. Their argument is essentially “so what if there’s a collision, the human review will catch it”. And to that I’d say that the same is supposed to occur for the no-fly list and we all know how that works in practice.

The mere accusal itself of possessing CSAM can be life ruining if it gets to that stage. More importantly, a collision will effectively allow warrantless searches, at least of the collided images.