This is all colonial suugo science . It is now an accepted fact that the oromos did not settle in the Somali peninsula or northern Somalia . These myths were spread by colonial writers due to encounters with somali nomads (such such as the above excerpt ). The oromos were not present in northern Somalia land before the somali nor is there any proof to show harlas lived there. The oromos came into contact with Somalis during their expansion in the 16th century .
The somali nomads often assign their land to other so called predecessors for two main reasons :
1.they incorrectly assume any sign of civilization ,meaning not nomadic in nature ,must be attributed to other ethnic groups ,when we know that there has always been civilization in the somali peninsula ,especially the coastal areas .furthermore ,a pastoral lifestyle could have been adopted by the Somalis due to the drying of the land.
2.somali nomads largely believe they are descendants of Arab sheikhs ,therefore any kind of ancient pagan settlements or ruins are automatically described as being of Oromo or assigned to some alien people.
Those ruins were most likely built by the ancestors of the somali before they accepted islam .
So the Samaales became Muslim and lost their ability to build stone buildings in the Axumite style? Remember that the Oromo fallacy came from I M Lewis' interviews with Northern clan elders and was apparently repeated by the Dhulbahante informants. This did not originate with the British. You REALLY need to read Sada Mire.