IIRC there was a teenage girl that lied about her father molesting her back in the 2000s. No evidence, just pure hearsay. Dude was locked up for like 10 years. Now imagine that on a greater scale with several videos and photos taken out of context.
Sorry, and as for "what is abuse", these are looking for exploited children. There are a lot of questions here, the morality of mass surveillance, the risk factors due to hash collisions (intentional or otherwise), etc, the one thing I doubt is an issue is a fuzzy line on "is this abuse"
I do think your comment is conflicting though. The fuzziness of 'is this abuse' is real because collisions can occur, mass surveillance never goes in reverse, etc.
Of course they have to be prevented from hurting people, to the best of our ability to do so, but punishment isn't going anywhere.
What is not a real concern is fuzziness in the question "is this image of child abuse". And I really don't want to continue a discussion where you try to come up with gray areas. A computer may not be able to tell the difference, but humans can.