I don't know where people would ever have gotten such an impression. Horners ourselves are literally
North-Sudanese. Our ancestors lived there before around 3000 BCE then migrated down to the Horn which explains why around 3000 BCE the Horn suddenly goes from being in the Paleolithic to there being cattle, goat, sheep and donkey pastoralism with grain collecting & processing, shepherd/hunting dogs, pottery, bows and arrows and all the other stuff like burial mounds, dolmens, rock-carvings, cave-paintings... no place makes that drastic a cultural shift that fast and the linguistic evidence strongly points to an origin in the Sudanese Neolithic:
The evidence that Proto-sahelian borrowed its words for βgoatβ from an already distinct ancestral beja language in the later seventh millennium6 supports the conclusion that the initial divergence of Proto-Cushitic into the Beja (North Cushitic) branch and AgΓ€w-East-South-Cushitic branches began before 6500 BCE -
History and the Testimony of Language by Christopher Ehret
So even as early 5,000-6,000 years ago at the minimum, Sudan was already looking like a Nilotic - Middle-Eastern intermixture on a genetic level and I reckon it clearly stayed that way eversince. Especially along areas like the Red Sea hills and what is regarded as
Nubia. All that happened is that new admixture elements got tacked on. Like historic period Egyptian, Islamic period Arabian and West-African ancestry carrying Nilotic elements.