God can speak in a way humans cannot. He can put signs in his speech that would indicate it's from him and beyond capability of humans. I think this is rational, as we see there is ranks to eloquence, and not everyone is capable of the same eloquence.
Therefore if God speaks in a way beyond all humans and challenges all humans and Jinn, to bring something like it if they don't believe it's revealed by God, I believe this is a legitimate challenge.
I also believe some of the sermons and prayers and visitations taught by Ahlulbayt (a) are beyond normal humans, but still, I can see Quran is MUCH higher in eloquence, form, and speech.
So people can't even replicate some of the works of Ahlulbayt (a) and bring something similar to it, let alone the Quran which is signs from God in form of speech.
I believe it's legitimate challenge, bring something like it or akin to it. Another challenge it poses which is fair, you claim it's not guidance from God, then bring something more guiding than it.
Both challenges are fair from my perspective.