They describe a process for third parties to audit that the database was produced correctly.
The story here is that there is a black box of pictures. Apple will then use their own black box of undeclared rules to pass things along to the feds which they have not shared what would be considered offending in any way shape or form other than "we will know it when we see it". Part of the issue here is that Apple is taking the role of a moral authority. Traditionally Apple has been incredibly anti-pornography and I suspect that anything that managed to get into the database will be something Apple will just pass along.
But if your problem is with NCMEC, you’ve got a problem with Facebook and Google who are already doing this too. And you can’t go to jail for possessing adult pornography. So even if you assume adult porn images are in the database, and Apple’s reviewers decide to forward them to NCMEC, you would still not be able to be prosecuted, at least in the US. Ditto for pictures of Winnie the Pooh. But for the rest of what you describe, simulated child pornography is already legally dicey as far as I know, so you can’t really blame Apple or NCMEC for that.
In any case it is likely the government would try to negotiate a plea to get you into some predator database to help fill the law enforcement coffers even if they have no lawful case to take it to court once they have your name in their hands.
References to Winnie the Pooh in these discussions are about China, where images of Winnie are deemed to be coded political messages and are censored.
The concern is that Apple are building a system that is ostensibly about CSAM, and that some countries such as China will then leverage their power to force Apple to include whatever political imagery in the database as well. Giving the government there the ability to home in on who is passing around those kinds of images in quantity.
If that seems a long way indeed from CSAM, consider something more likely to fit under that heading by local government standards. There's a country today, you may have heard of, one the USA is busy evacuating its personnel from to leave the population to an awful fate, where "female teenagers in a secret school not wearing a burqa" may be deemed by the new authorities to be sexually titillating, inappropriate and illegal, and if they find out who is sharing those images, punishments are much worse than mere prison. Sadly there are a plethora of countries that are very controlling of females of all ages.
If this system didn't exist, nobody would have to trust Apple.
> you would still not be able to be prosecuted
But I wouldn't want to deal with a frivolous lawsuit, or have a record in the social media of being brought CSA charges.