Where did this figure come from? Some muslim group wanting to join the grievance parade?
I wouldn't question 15%. The number is derived from the fact that about half the areas slaves were taken from had Muslims. Africa was an early victim of Islam. You can find much higher numbers. Though they were a minority. There are a few Arabic, which are assumed Muslim, names on Revolutionary War muster roles. Slaves were not necessarily practicing their faith, particularly in multiple generations of slaves. Does any of this make the case of some sort of contribution to American culture, of course not.