Diamond isn't really talking about the importance of Cajamarca for Western civilization. He brings Cajamarca up more for what the events there tell us about why Western countries were able to conquer other peoples.
Cajamarca was important to Western civilization in that it was the site of the battle that allowed the Spanish to take control of what had been the Inca Empire. This gave the Spaniards control of the huge silver mines of Peru and enriched the Spanish Empire.
But Diamond really isn't talking about that. He is trying to show how the Spaniards' ability to defeat the Incas demonstrates the major trends and factors that he says have determined the general course of human history. He says that the Spaniards' victory shows the importance of the "guns, germs, and steel."