When I pursued creative writing in my teens and early 20s, it became clear to me that originality is extremely difficult. I am not entirely sure I have ever had an original thought--every idea I've put to paper thinking it was original, I later realized was a recombination of ideas I had come across somewhere else. The only exceptions I've found were places where I had a fairly unusual experience which I was able to interpret and relate, i.e. a unique interaction with the world.
Perhaps more importantly, LLMs do not contain any mechanism which even attempts to perform pure abstract thought, so even if we accept the questionable assumption that humans can generate ideas ex nihilo, that doesn't mean that LLMs can.
Unless your argument is that all creative writing is inspired by God, or some similar "external" source, then clearly a closed system such as "humanity" alone is capable of generating new creative works.
