There's a lot of misconceptions about AI image generators in here.
They can indeed generate "truly new images", ask an image generator for an image of something that definitely doesn't exist in its training set and it'll likely be able to come up with something like that for you. Most importantly for purposes of this discussion, you don't actually need to have any images specifically of child abuse in a model's training set in order to train it well enough to produce images of child abuse. Train a model with a bunch of regular porn and a bunch of ordinary images of children and I expect it'll figure out how to make images of children in sexual situations if you ask it to.
This has been known for years. These AIs are capable of "understanding" the things they're trained on and creating novel interpretations of those things.
There was an article recently that showed if you trained many generations of AIs on just the outputs of previous generations you got degraded performance over time, but that's a pretty specific scenario that doesn't match what's being done in real life. In real life synthetic training data (ie, AI-generated training data) can be very useful for expanding the capabilities of AI as long as it's well-curated (humans need to select good outputs and ensure they're described correctly) and ideally has some of the earlier training set's original data included as well.