I often hear this statement made—mostly by white supremacists— presented along with other supposed evidence that Sub-Saharan Africans were primitive and uncivilized, like failure to domestic animals and crops (which I know isn't true). I know that Mali, Somalia and other places that converted to Islam had written language and even universities/libraries, such as the one in Timbuktu. It's also my understanding that very few writing systems, around three I believe (cuneiform, Mesoamerican script and Hanzi) were created independently, so I take it that the geographic isolation from the Mediterranean and the cradle of civilization is the reason why there was no written language in Sub-Saharan Africa. Is this correct?
I know that Mali, Somalia and other places that converted to Islam had written language and even universities/libraries, such as the one in Timbuktu.
I think you answered your own question? Folks in areas of Islamic influence were writing in Arabic long before Europeans show up.
Also, they sometimes used Arabic letters to write in their own language. Systems like that are called Ajamis, and we have ajami manuscripts from Timbuktu
Outside of that, you also have examples like Ge'ez in Ethiopia, which was adapted from an alphabet system developed in southern Arabia. Proto-Ge'ez dates back to maybe the 9th century BC.
Similarly, the Nubian kingdoms of the upper nile (modern sudan) had a system of heiroglyphics derived from Egyptian heiroglypics, and a separate alphabet derived from Egyptian demotic script. This is known as the Meroitic alphabet, and was in use up until about 600 AD.
Otherwise, much of Central, Southern and Eastern Africa (except for the Swahili coast) did not have writing systems prior to European contact. And we don't have evidence of writing systems independently invented in Sub-Saharan Africa (obviously excluding Egyptian heiroglyphics in this construction).
Though, on the other hand, scholars have become more and more interested in how visual symbols/patterns in African art have been used to communicate meanings. For instance, how Adinkara symbols on cloth communicate meanings among Akan peoples in Ghana. Or how shapes and colors communicate meaning in Zulu beadwork. These systems of symbols and meanings can be thought of as a sort of visual language, though they take very different forms from what is usually considered writing.