The T-Y44591 timeline is between 3000 ybp and 1850 ybp.Your argument is not without merit. However, genetic genealogical evidence cannot be used to support it yet unless we see more diversity in the Somali T lineage TMRCA. As things stand, we only have that founder effect.
It is possible that he was part of a group migration event, possibly even with the J haplogroup Darood individuals who have distant Arab ancestry. However, the migration date would have to be later than the current TMRCA date of the Somali T lineage as a century or two would have to pass before a 'clan' was formed etc. Hence, the conquest theory does not stand in my opinion. There is also no oral historical recollection among Somalis of a foreign group of men imposing themselves upon a host population.
My preferred theory is that the individual in question is connected to the introduction of a new way of subsistence (Camels) or trade. There are historical records of Arabs trading in Somalia before the adoption of Islam. Moreover, the TMRCA date matches up with Eret's hypothesised introduction of Camels into Northern Somalia from Arabia. He was probably not alone but his lineage proved successful for one reason or the other. We might never know why but I am pretty certain conquest is not one of them.
What do you hypothesise as the cause for the migration? I think @Cuneo posits it as the fall of the Thamud. Or do you think it was just a trader from Arabia?