Assuming this is a work of fiction, more creative than historically / geographically accurate, we need to understand what kind of cartographer we are talking about. Two hypotheses:
- an "amateur" cartographer, not a real scholar, (and not in contact with a network of scholars), with little travel experience but a certain degree of knowledge of geography (but not so deep), that maybe had misunderstood maps of that time, in which the strait of gibraltar is a very "thin" sea strip (
example);
- a cartographer that wanted to represent conceptually the unity of the Western Muslim lands after their conquest.
Otherwise the map is not very accurate, since we have many examples of our ancestors' knowledge of the geography of the Strait of Gibraltar: for example, the Umayyad conquered much of the Iberian peninsula as early as 711 AC , not to mention, long before, the Carthaginian attempt to conquer Rome passing through the territories of Hispania and Gallia (second punic war, 218-202 BC).
In any case, as I said, I really like this map.