That is because so many self-styled conservatives are in fact radical secularists who espouse the values D'Souza tactfully (or perhaps tactically) associates with the Left. See, e.g., Rudy Giuliani.
You're absolutely right. In fact, I would make the case that what we know as "neo-conservatism" is basically a political movement grounded in radical secularism. That's why its proponents see the United States as nothing more than a tool for establishing a secular, globalist empire.