A lot probably was written by those Medieval groups but most of it is likely lost now or sitting in some medieval library somewhere like in Harar and yet to be seen and put online or in University databases, basically. When you read up on early modern Somali sheikhs, wadaads and townspeople you will notice there was clearly quite a bit of literacy and people writing things down but due to everything being on perishable objects like paper we've lost a lot to the sands of time, unfortunately.
Take this guy:
Shortly after arrival, I sent my Salam to one of the Ulema, Shaykh Jami of the Berteri Somal: he accepted the excuse of ill health, and at once came to see me. This personage appeared in the form of a little black man aged about forty, deeply pitted by small-pox, with a protruding brow, a tufty beard and rather delicate features: his hands and feet were remarkably small. Married to a descendant of the Sherif Yunis, he had acquired great reputation as an Alim or Savan, a peace-policy-man, and an ardent Moslem. Though an imperfect Arabic scholar, he proved remarkably well read in the religious sciences, and even the Meccans had, it was said, paid him the respect of kissing his hand during his pilgrimage. In his second character, his success was not remarkable, the principal results being a spear-thrust in the head, and being generally told to read his books and leave men alone. Yet he is always doing good “lillah,” that is to say, gratis and for Allah’s sake: his pugnacity and bluntness—the prerogatives of the “peaceful”—gave him some authority over the Amir, and he has often been employed on political missions amongst the different chiefs.
He was a very prominent and well-read scholar and political figure from Harar of Somali descent and his clan frequently intermarried with the Emirs of Harar with that generation's Emir's first wife being of this man's tribe (Bartire) and the daughter of their chieftain, yet I had to find out about him through a British text:
He no doubt left behind writings throughout his life and there were probably records of him somewhere. Just yet to be found or lost forever. It's unbelievable to think how many interesting Somalis throughout history there have been whom we know close to nothing about.