Genetic studies debunk this. The people who call themselves Somalis today exclusively mix amongst themselves at the exclusion of every other group. Ethnicity is literally defined by the in-group/out-group tendencies and the facts on the grounds say we were highly endogamic, with Ethiopians mixing across divergent speech groups if they were in geographic proximity creating a gradient, while Somalis in Ethiopia were the only group identified to break that tendency, at the same time, showing they highly mixed with other Somalis. Somalis simply mixed with Somali clans while totally not with other ethnicities -- and this has been a thing since we came to the region, given how we retained not only homogeneity but also very pristine Cushitic DNA no other group in the Horn of Africa retained due to mixing to the degree of Somalis. This has been a several thousand-year process and has nothing to do with colonialism.
From Hammeren et al., 2022:
"At K=11 another East African component appears maximized in the Somali populations and as such might represent Cushitic related ancestry."
"The analysis using FEEMS recapitulates expected natural barriers to migration such as the Red Sea, the Gulf of Aden, the Persian Gulf, and the Sahara Desert. In addition to clear geographical barriers, we also see
evidence of linguistic and cultural barriers. One obvious example is the
low migration rate between the Ethiopian Somali and the other Ethiopian populations, and as expected high migration rate inferred between the different Somali groups."
Somalis had high intra ethnic but inter-clan mixing:
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Ali et al., 2020, showed how the Cushitic DNA has been stable and retained among Somalis ever since the divergent components mixed, meaning only one thing, not only did we mix exclusively with groups already matching the DNA proportions before coming to the Horn of Africa, but that we retained those dimensions to en extraordinary high degree since that signature genesis:
"The similarity of individuals is apparent, which presumably reflects very ancient admixture events and a unification process through endogamy."
"We observed that a vast majority of our study individuals carried similar proportions of genes from those ancient populations, which possibly can be explained by the fact that ethnic Somalis have a strong genetic unification by endogamy, due to the custom of marrying only within the limits of their ethnic group, in addition to consanguineous practices."
"However, recent evidences based on Y-STR haplotypes studied on Somalis from diverse geographic locations and clans suggest that ethnic Somalis are largely homogenous, supporting the representativeness of our samples for the larger Somali population."
This tells you as a man with no prior knowledge on this subject that there could never have been intra-ethnic diverse structuralism.
Hodgson et al., 2014 came to the same conclusion by observing the groups of the region:
"We then used the ADMIXTURE results to inform subgroup formation. At K = 12, the Amhara, Tygray, Oromo, and Afar all have similar proportions of non-African ancestries that differ from that seen in the Ari and Somali (Figure 2). This observation suggests a geographical structuring between the Amhara, Tygray, Oromo, and Afar in the Ethiopian highlands,
the Somali in eastern Ethiopia and the Somalia lowlands, and the Ari in the southwestern Ethiopian Rift."
There was always Somali-specific ancestry structuralism no matter what clan or geographic placement at the exclusion of other groups. Somalis break the tendencies characteristic of the whole region basically only conforming to their own ways. So again, this is impossible without an ethnic conception... This discrimination is the quintessential definition of a coherent group you can ever find and it had nothing to do with colonialism.