The best line from your link:
Possible explanations for this decline include hybridization with sub-Saharan Africans, dysgenic decrease in the more educated Muslims employing birth control as suggested by Meisenberg, the Muslim religion not fostering critical thinking, and the intellectual contributions being both exaggerated and made by non-Muslims. Total denial of the Islamic tradition of first cousin marriage shows the bias of the authors or their astounding incompetence as academics. Marriage of first cousins over generations apparently makes you dumb and dumber.