Theme? Hardly. It was a single sentence offered as one single example of the book's contents. It appeared halfway through the article and well after the authors had twice explicitly identified the topic of their book: the events of the civil war as viewed and analyzed through the tools of economics or, as the title says, "The Economics of the Civil War."
Face it, mac. The questioned asked you for the topic of the article and you picked the wrong sentence as your answer. I will further note the high likelihood that you were led to choose the sentence that you did out of a strong personal bias against the south and a desire to see it criticized to the point that you will seize upon a criticism and emphasize it to the neglect of the vastly more relevent material amongst which it sits. This suspicion is supported by the fact that you chose it from a location halfway through the article's text and in spite of the authors not once but twice informing you that the correct answer was another one located in the opening paragraphs.
Hmm, not too good with discrete concepts are you?
Perhaps you should undergo some thematic appreciation testing to determine what the problem might be. (lol).