The spreading of Islam was a little more complicated than history books generally have it. Yes, they arrived in Spain and rapidly subjugated it less than a hundred years after Muhammad's death in 632 (first arrival in 711, to be exact), but that does not mean all the desert tribes in Northern Africa had been converted, and there have indeed been a number of westward waves of Arabian tribes into Northern Africa, clashing with the local tribes and contributing to the spread of Islam and the Arabic language, some as late as the eleventh century iirc. In the case of the Almoravids, they were nominally Muslim, but in such an ignorant and clueless way that their learning more precisely what Islam was about must've been rather like a conversion, and that did indeed happen around the mid-eleventh century, which is when the main parts of the plot happened.