The class is called social and political philosophy. That means it is a philosophy class about the state and the kinds of societies that have states. The Diamond reading was meant to identify some of the features of these kinds of societies.
In particular, Diamond notes the mutual relationship between population, agriculture, and the state. The state emerges along with large populations and large populations can be sustained only by agriculture.
It was not always this way. Most humans have lived in what Diamond calls traditional societies without political authority. Traditional societies are more equal than state societies in at least two dimensions. First, there is no hierarchy of authority: no one is in a position to issue rules and orders that everyone else has to obey. Second, there is less material inequality.
While it would be a stretch to suggest that all, or even any, of the philosophers we are going to read were aware of these facts, it is nonetheless the case that they are almost all preoccupied with these two features of state societies: political authority and material inequality.