That's probably incidental, horrible as it is. Models don't need training data of everything imaginable, just enough things in combination, and there's enough imagery of children's bodies (including non-sexual nudity) and porn to generate a combination of the two, same as it can make a hybrid giraffe-shark-clown on a tricycle despite never seeing that in the training data before.

The biggest issue here is not that models can generate this imagery, but that Musk's Twitter is enabling it at scale with no guardrails, including spamming them on other people's photos.