The bottom line was written at the beginning of the book, and he spent the rest of the book providing arguments for it.

It seems to me that the original quote is an explicit statement that that is what he is going to to. As is, even more explicitly, the mission statement on the top page of that website. An extract:

History isn't what happened, but the stories of what happened and the lessons these stories include. ... We cannot simply be passive. We must choose whose interests are best: those who want to keep things going as they are or those who want to work to make a better world. If we choose the latter, we must seek out the tools we will need. History is just one tool to shape our understanding of our world. And every tool is a weapon if you hold it right.