Then every employee of openai should scrape the weights of GPT-4 and train their own neural nets. That would not be a derivative work and also be free use under this logic.
The employees would still have a contractual responsibility about their use of the model weights.
If the model weights encode other copyrighted works literally enough then they may be copyrighted by the author(s) of the works, that is a gray area. But the training process itself is not authorship and doesn’t confer copyright.