That’s because the example they gave either a) combines two concepts the AI already understands, or b) adds a new concept to another already understood concept. It doesn’t need to specifically be trained on images of possums wearing top hats, but it would need to be trained on images of lots of different subjects wearing top hats. For SD the top hat and possum concepts may be covered by the base model datasets, but CSAM isn’t. Simply training a naked adult concept as well as a clothed child concept wouldn’t produce CSAM, because there is nothing in either of those datasets that looks like CSAM, so it doesn’t know what that looks like.
That’s because the example they gave either a) combines two concepts the AI already understands, or b) adds a new concept to another already understood concept. It doesn’t need to specifically be trained on images of possums wearing top hats, but it would need to be trained on images of lots of different subjects wearing top hats. For SD the top hat and possum concepts may be covered by the base model datasets, but CSAM isn’t. Simply training a naked adult concept as well as a clothed child concept wouldn’t produce CSAM, because there is nothing in either of those datasets that looks like CSAM, so it doesn’t know what that looks like.