https://tommyhinks.com/2009/02/10/preetham-sky-model/
https://tommyhinks.com/wp-content/uploads/2012/02/1999_a_practical_analytic_model_for_daylight.pdf
Rather than stolen from Mr. Preetham, it's much more likely this fragment is generated from a large number of Preetham algorithm implementations out there, eg. I know at least Blender and Unreal implement it and probably heaps of others was well.Nobody is going to sue you for using their implementation of a skybox algorithm from 1999, give us break. It's so generic you can probably really only write it in a couple of different ways.
If youre worried about it you can always spend a day with Claude, ChatGPT and yourself looking for license infringements and clean up your code.