Denying that atheism by itself constitutes a belief system is different from denying that atheists have a belief system. The first point is semantic, and atheism itself is indeed just a negative point, the denial of theism.
But those atheists who aren't nihilists do have some positive, godless worldview. It's just an empirical question how many atheists share which particular atheistic belief system. The most common one in North America, Europe, and Australia, I think, is some combination of secular humanism (Enlightenment liberalism) and neoliberalism (consumerism).