I don’t know who Kenneth Lamb is, but the information is wrong. The Luo tribes are not arab. The Luo actually left their ancestral homeland because of arab conquest. The Luo are black Africans and very dark skinned.
arabs are notoriously racist. They don’t like black people.
However, there's no way to prove that some of the male line in that family were NOT arabs. The arabs did inter-marry...or should I say, have children with blacks.
There's also the fact that the female line doesn't count with islamic blacks, and to be able to call yourself arab held certain advantages.
The other argument, that arabs are not black falls down with the fact that the arabs in that region were Omani.