The salient bit:
> A quick note to everybody who says 'calculate the hash locally':
> A) Photo fingerprinting algorithms are usually not included in clients to prevent the development of circumvention techniques.
> B) Humans need to review to prevent adversarial reporting.
Further down the thread, they discuss further that PhotoDNA and similar algorithms are, in particular, not resistant to being tricked. Until they are, it's another layer to prevent adversaries from fully decomposing how they're hashing it, which is a good thing.
It's a trade-off, for sure, but this is a hard problem with no cut and dry answer. Facebook already has the images in question, because you have to be sending them to somebody in order to mark them as NCII. Asking yet another Facebook service to look at them without storing them seems reasonable to me. You may disagree.