Plus he remains the most successful poet-king in history, well maybe an argument could be made about David, but the authorship of the OT is problematic, to say the least. There is no doubt this is a formidable figure, and his is a formidable poem. I tend to think every poet is in some manner. What we can perhaps say with certainty is that the dogmatic life of Muhammed, though accurate in parts, is to be taken with a grain of salt. Frankly, who the hell cares.
Artistic freedom is absolute, and strong authors know that intiutively.
And Rushdie does not even claim anything but fictional status for the book. Maybe Dante wrote it because he hated a church that bred such bishops with a vengeance. Maybe Donne sold out to the protestant church, stifled his religion. Maybe Paradise Lost is the cry of a blind man who has been cast from the realm of light. Maybe God is dead.