You are being very silly. North Africa is oriented East-West in a way that allowed it to participate in the easy exchange of flora, fauna, culture and technology with the greater East-West zone that extends from Portugal to China. Saharan Africa is dominated by the Sahara Desert, which is a huge barrier to exchange. Sub-Saharan Africa is oriented mainly North-South, in comparison with the greater Mediterranean exchange region the Diamond outlines.
North Africa is included in Diamond's Mediterranean exchange zone. And that zone extends out of Europe to the Fertile Crescent, at the very least, and arguably to India and China as well. Africa's two zones are separated by the largest and most inhospitable desert on earth. There is no advantage in having two.
That is in fact the core idea Diamond is expressing.
Domesticatable flora and fauna from a huge and ecologically diverse geographic region were easily diffused around the entire zone. The difficulty of migrating horses or sheep across the Sahara, or turkeys and llamas across the Darien Gap, is much much higher. Sub-Saharan Africa started with few large animals suitable for domestication, and importing new ones was comparably difficult.
https://upload.wikimedia.org/wikipedia/commons/thumb/6/61/Eq...