from the article:

AI-generated CSAM may also present other challenges not raised by the government in this appeal. For example, researchers have found that "a major dataset used for training image-generating AI contained hundreds of CSAM images." Thus, unlike the virtual CSAM at issue in Free Speech Coalition, the market for AI-generated virtual CSAM images may impact the demand for actual CSAM on which the AI models can train.

I said risk, not that it definitely would happen. Caution, do you know about that? Or reading comprehension.
It's the same as when a chemical isn't proven safe and businesses are not allowed to add it to your food. It may be completely safe, but just in case.
If the benefit of using something outweighs the risk it would be reasonable to allow it, but we are talking about something with no proven benefit other than letting pedos enjoy themselves.
Not to talk about the re-victimization of CSAM survivors because there are cases of victims appearing in AI generated material because of the training using the original rape media. But I guess you don't care and would say they just look alike or some excuse.