The wording is vague, but I believe it's also in reference to the number of slaves shipped. One of the Wikipedia page's sources is "Islam's Black Slaves" by Ro

The wording is vague, but I believe it's also in reference to the number of slaves shipped. One of the Wikipedia page's sources is "Islam's Black Slaves" by Ronald Segal. He estimates the total number of African slaves shipped to the Muslim world at 11.5-14 million. Raymond Mauvy estimates it at 14 million shipped. I wasn't able to find a good estimate for total number ever enslaved for either the Americas or the Muslim world.