IIRC it was something like a fraction of a fraction of 1% that was CSAM, with the researchers identifying the images through their hashes but they weren't actually available in the dataset because they had already been removed from the internet.
Still, you could make AI CSAM even if you were 100% sure that none of the training images included it since that's what these models are made for - being able to combine concepts without needing to have seen them before. If you hold the AI's hand enough with prompt engineering, textual inversion and img2img you can get it to generate pretty much anything. That's the power and danger of these things.