This sentence is the last sentence of Chapter 4 in the book. In this chapter, Zinn is arguing that the American Revolution was not really the noble effort to create a fair and democratic society that we tend to think it was. Instead, he argues, the Revolution was mainly a way of replacing one elite (the British one) with another elite (the colonial or American one). The Revolution, then, did not really do much to change the experience of the working class and the poor.
In this chapter, Zinn argues that the Founding Fathers were really just a set of elites who were interested, as all elites are, in maintaining their own power and keeping the lower classes in their place. He says that the American elites used democratic rhetoric to get the lower classes on their side but did not really believe in the rhetoric.
Zinn points out that 69 percent of the men who signed the Declaration of Independence had held office under the British government. This was not a revolution, just a changing of the top layer of society. Zinn closes the chapter by saying that the coming of the Revolutionary War led to a draft in Boston, but one that the rich did not have to participate in. In other words, the American “patriot” elites (he says) were simply treating the lower classes just as tyrannically as the British had. Their tyranny, he argues, was no less tyrannical just because it was done by people born in the colonies.
What this sentence means, then, is that the Founding Fathers were, in Zinn’s mind, no better than the British elites who had ruled before them.