We aren't disagreeing because that's not what I was addressing in the first place. The comment I'm responding to, from Dave, reads:
In that case probably the strongest argument is that if it were legal, many people would get off charges of real CSAM because the prosecuter can't prove that it wasn't AI generated.
Emphasis added. The premise of the scenario is that possession of such images (ie, AI-generated CSAM) is not illegal. Given that, for purposes of argument, it follows that this would indeed be a valid defense. You'd need to prove in court that the CSAM pictures that you're charging someone with possessing are not AI-generated, in that scenario.
If you want to have a wider discussion of whether AI-generated CSAM images should be illegal, that's a separate matter.