In the US if I get caught selling you a gram of pure cocaine I get the same punishment as I would if I sold you a gram that’s only 20% pure. If I sold you a gram of some random powder and told you it is cocaine I am likely to be prosecuted all the same whether I knew it was fake or not. That aside, the “fully synthetic CSAM with no children involved at all” idea relies very, very heavily on taking the word of the guy…
Although currently it is not entirely clear whether mere possession should continue to constitute a crime by itself, or it should require actual child abuse (because the former used to imply the latter).