Chinguetti - holy city of Mauritania

"Being the seventh holiest city of Islam, the old city of Chinguetti is home to numerous bibliothèques, containing thousands of priceless books and documents dating back to 1000 AD, perfectly preserved because of the dry desert conditions." Seventh holiest city in Islam? I came across this extraordinary claim while browsing Michael Palin's website, looking at pictures from his journey through the Sahara. This gem accompanied a picture of the Bibliothèque Al Halott. What was the basis for this ranking I wondered? Was there somewhere out there a league table of holiness inspired by some medieval muslim new-labourism gone mad? A quick google search confirmed that this was an oft-repeated claim, that Chinguetti is a UNESCO world heritage site and the most visited city in Mauritania (not saying much), but revealed no further evidence of the existence of such a list, ranking in order of sanctity the metropolises of the medieval muslim world...