The answer to this can be found on p. 382 and following pages in the paperback edition of the book. On p. 382, Diamond tells us that
…languages have tended to evolve along with the people who speak them.
Diamond is saying that people have evolved to have different physical characteristics. We identify people who live in Africa (the topic of Chapter 19) as black, Khoisan, or “white” (which is what Diamond calls the people of North Africa). These different groups tend to speak different families of languages. For example, Diamond says, blacks speak Nilo-Saharan and Niger-Congo languages while Khoisan speak Khoisan and whites speak Afro-Asiatic languages.
This is relevant to Diamond’s argument because he uses the geographic distribution of languages in Africa to tell him how humans spread across the continent and came to be distributed as they are. Using studies of language, he concludes that southern Africa had originally been inhabited by Khoisan people and Pygmies. The Bantu-speaking black people then came out of their ancestral homes and colonized most of southern Africa, pushing aside the Khoisan and Pygmies and leaving them only in a few marginal places that were not good for the Bantu-speakers’ agriculture.
Thus, the way in which language develops along with the people who speak it is relevant to Diamond’s argument in Chapter 19.