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.