I think you have a good point. But don't both sides (astrologers, sorcerers, Jinn, etc evil side and good side)claim to be connected to the divine? If so, the one truly connected should be able to bring the best writing and most eloquent, because the divine can?
So the Quran challenge kind of has layers.
If you are atheist, get some humans to bring the likes of it... since it's just works of humans in your view.
If you are a believer in God and his Prophets, then you should believe God would reveal the best writing and if it's not, then you can bring the like of it too or something better (something revealed by God).
If you are soothsayers type pagan religion, astrology, etc, then you claim your Angels are connected to the divine or are gods or whatever and you have connection to them or some people do - then bring a writing similar if it was evil forces that made the Quran from Jinn or humans.
If you can bring something better in guidance as well, I think that would be fair to say that challenge in Quran is refuted. If Quran allows you to bring something more in guidance, I think that's fair for people who believe in God. For those who don't, then at least bring something similar to it. For those who believe in God, either or, you can bring something like it or at least if you can't, bring something more guiding then it.
But if neither, then that's a problem.