Previous research has shown that if you search the entire training dataset for the image most similar to one generated by a generative model like this, the images actually look pretty visually different. To a non-expert, they'd say the images are similar, but neither copied the other.
There are two inputs here: a collection of photos and a model. It's not being derived solely from the original photos. The scientist had a contribution as well, which cost him money and time to develop. As long as it's not wholesale copying and there is significant creative input (selection, changes, recombinations, etc) then I think it should be ok.
In legal terms, if your AI model is "sufficiently transformative," you don't need to worry about anything like that. But if your AI model is overfitted and is just memorizing, then yes, you're right.
The way how these AIs work is that they have memorized aspects of celebrity photos and then recombine them as needed. That means if your sketch looks in any way like a celebrity from the training data set, then the AI will likely reuse parts of those photos, which would make your generated texture a derived work, meaning you'd have to pay royalties to the celebrities in the dataset.