Fes university at least has always been an institute of higher education, which at its foundation as far as I'm aware really entailed being part of a mosque (rather as hospitals in mediaeval Europe were all attached to monasteries). If you only include establishments following the European model of university then "oldest university in the world" becomes a bit of a nonsense category, rather like "the biggest cybercrime since history began" or "the best Australian Rules football team in the world".