Every book of history is more than just a retelling of events -- each author retells events in such a way as to try to make a given point. In this book, Egerton is trying to make a point about Prosser's motives and about the society of the time.
Specifically, Egerton wants to argue that Prosser's rebellion was not so much a racial rebellion as it was a class-based rebellion. Egerton believes that Prosser wanted to enlist working class whites to fight alongside with blacks. The enemy would not be defined by the color of their skin. Instead, the enemy was supposed to be the merchant class of whites who profited from the labor of the workers.
Egerton is trying to prove this point -- enough so that he chooses to cite evidence to support it and he downplays evidence that would suggest that the rebellion really was racial.
In short, then, Egerton is trying to argue that Prosser's rebellion was one that came out of a form of class consciousness rather than out of racial animosity.