You can "very much doubt" it all you want but the fact of the matter is that the ancient Israelites spent hundreds of years in Egypt and were slaves. There were black slaves as well. It is entirely possible, indeed, I'm going to suggest very likely that the Cohen gene could have aslo found its way through Kohath's heirs apart from Aaron, in the black population. You rejecting this process is unreasonable and unwarranted. You don't reject other equally reasonable alternative explanations and this is clearly one.
I reject it because the Lemba seem to be too black for that. Even if there were Negro slaves in Egypt and Jews mixed with them I still wouldn't expect them to be so black as to be indistinguishable from sub-saharans. If the Lemba looked like mulattos or so I would consider it.
Reconsidering, if there was a large group of Negro slaves in Egypt and Jews mixed among them in a manner similar to that which might have happened in sub-saharan Africa it could be a remote possibility. It it strikes me as the most unlikely possibily. It's quite convenient that the only black Jews would have chosen to emigrate to sub-saharan Africa than anywahere else. It's also convenient that none of the white Jews chose to emigrate to sub-saharan Africa. I just don't buy it.
This is eminently reasonable.