Somalis weren't 100% somali that long ago. Even during the futuh the inland somalis in modern galbeed still had many Waaqist. Ahmad Gurey converted the remaining hinterland Waaqist somalis to islam as well as many Habash. 20% of Amhara are Muslim now thanks to Ahmad Gurey. As well as near all lowland horn.
My Jiini hunch is - Somalis were already converted Arab tribe who intermixed with citizens of the Ajuraan empire- the locals- and thus Somali was born.
Like the Arabs who settled in Mombasa -and intermixed language with the locals to make it Swahili