There isn't any ancient texts stating that. People create false traditions to make themselves feel better.
This source perfectly debunks the false traditions created by the Benadiri community claiming that they founded and built Mogadishu when in fact, they got permission from the local native Somalis to settle in one particular neighbourhood and paid tribute to the local Somali rulers just to stay. On top of that, the native Somalis retained their numerical and political dominance in Mogadishu. The Muslim immigrants didn't found or rule shit, same applies for Zeila and other coastal cities across Somalia.
These Muslim immigrants eventually became assimilated and intermarried quickly and became Somalized Arabs. There is a reason why your people speak Somali today.
As for "Sarapion". It wasn't called Sarapion. The Greeks simply named every city they have traded with to establish records. They done that with Indians, Azanians, Arabs, Persians and other coastal provinces around the world. However, their records explain how ancient Somalis operated and it's no different to today.