The answer to this can be found in Chapter 3. There, Diamond discusses the various factors that allowed the Spanish to defeat the Incas. Writing is one of those factors. Writing was important because it allowed for a great deal of information to be transmitted and stored very accurately. This was not something that could happen without writing. Written materials encouraged Spaniards to come to the New World. Written materials told them how to get there. The lack of writing meant that the Incas could not get good information about the Spaniards from other Native Americans who had contacted them.
Thus, writing was an agent of conquest because it allowed literate people to have much more information at their disposal than nonliterate societies did.