Artificial CSAM is constructed from real CSAM,
I would not be that sure. 30 years ago it was possible to create photorealistics images with renderes (ok, you needed days of rendering probably)
It was possible to create fake xxx images combining the image of the face of a person (famous actor/actress, common guy/girl) with a body from a image scanned from a xxx magazine in a way that it looks like an original image (there were entire subgroups on Usenet dedicated to this).
I bet any decent AI image creator could now do way better than what was done by hand 30 years ago.
to the point that real CSAM victims have sued people who generated material resembling the victim.
Also nothing new. Do you know the origin of the "Every protagonists of this story is a finctional charaters and [blah, blah blah]" you see at the end of every movie ? Basically same problem you cite.