This is what happened with Somali paternal frequencies (in ELI5 terms):
A group of different paternal lineages lived together in the Eastern Horn (proto-Somalis). Likely E-V6 (and other smaller versions of E-V1515 (E-CTS10880 on yfull), various E-V22*, different kids of E-V12* and E-V32, E-Y18629 specifically, E-M34, E-V16, and even likely A-M13 and potentially even old non-recent Arab versions of J1, then this alpha carrying E-Y18629 started alphamaling all the other ones and dominating the frequency squeezing all the other ones out.
Then around 2K YBP a male ancestral to T-BY181210 joined the Somali ethnicity. Perhaps he already had a small clan by that time and then joined in on the patriarchal culture pumping his lineage to relatively high frequencies. Likely he did not come alone, but his lineage was more successful. perhaps some of those J1s in Afars came with him.
I don't think E-Y18629 and T-BY181210 are representative of Somalis total origins. They squeezed out many of the other ones.
I am more inclined towards Bulliet's work in which he stated that Socotra might have been the staging point from which camels, short-honed zebu cattle were introduced into the Horn around 2500-1500 years ago by Semitic migrants. Northern Dir subclans that possess T-BY181210 at a high percentage migrated West from North-Eastern Somalia. Zeila, at its peak, was not yet our territory fully until a few centuries ago.
The Afars and our Semitic friends across the sea probably interacted more considering the ease of commute between their territory and Yemen. This is reflected in their partial Semitic paternal ancestry which has two sources in my opinion, a main ancient Ethio-Semitic source and a more recent Yemeni one. Nonetheless, this is all hypothetical until a couple of their J carriers are analysed on YFull.