Even if you do a content aware hash where you break the file into chunks and hash each chunk, you still wouldn’t be able to magically recreate the hash of a CP file without also producing part of the CP.
It's the weights from the middle of a neural network that they're calling a "hash" because it encodes and generates an image it has classified as bad. Experts have trouble rationalizing about what weights mean in a neural network. This is going to end badly.
If this was a hash then it would be as the parent describes, this is at best a very fuzzy match on an image to take into account blurring/flipping/colour shifting.
It's vastly more likely that innocent people will be implicated for fuzzy matches on innocuous photos of their own children in shorts/swimming clothes than it is to catch abusers.
The other thing is, when you have nothing to hide you won't take efforts to hide it - meaning you'll upload all of your (completely normal) photos to iCloud without thinking about it again.
The monsters making these images know what they're doing is wrong, so they'll likely take efforts to scramble or further encrypt the data before uploading.
tldr; it's far likelier that this dragnet will only even apply to innocent people, than it is to catch predators.
All this said, I'm still in support of Apple taking steps in this direction, but it needs far more protections put in place to prevent false positives than this solution allows. A single false accusation by this system, even if retracted later and rectified, would destroy an entire family's lives (and could well cause suicides).
Look what happened in the Post Office case in the UK as an example of how these things can go wrong - scores of people went to prison for years for crimes they didn't commit because of a simple software bug.
The ones that make national news from big busts do, because the ones that don't get caught much sooner and only make local news, because Google and other parties are have automatic CSAM identification online already (server side, not client side, AFAIK), and are sending hits to Homeland Security.
A malware will definitely be created, almost immediately, that will download files that are intentionally made to match CP - either for the purposes of extortion or just watching the world burn.
I'm usually sticking my neck out in defence of more government access to private media than most on HN because of the need to stop CP, but this plan is so naive, and so incredibly irresponsible, that I can't see how anyone with any idea of how easy it would be to manipulate would ever stand behind it.