How is a human supposed to distinguish that a visual derivative (a low res sobel filtered image, presumably) of ordinary, lawful, adult pornography isn't child porn when the system has already identified it as such?

I agree that using real child porn is an attack too, but at least in that case you could say the system was doing as designed (even though what its doing shouldn't be something that we want) ... but it's not even guaranteed to do as designed.

"Amazon has people transcribing audio in Costa Rica, India and Romania." according to Engadget. So it would be safe to assume Apple does something along those lines, I can't remember where Siri transcripts have been sent when there was the Alexa controversy.

From there it's an entry with law enforcement and you need to find a way to convince them that the images are your children.

And if data isn't cleared correctly and you have another run in with law enforcement, there will always be this picture-stuff.