The same can be said of "theism". That encompasses everything from the Pope to an animist witch doctor.
I also think it's fair to say that atheism was fundamental to Marx's, Nietzsche's and Sartre's philosophies.
atheism can stand alone and not as a negation of theism.
Marx was the ultimate rational materialist. Nothing was left to supernatural wrangling. His was an analysis of the material and how it defined history. His revolutionary concept was that labor can be assigned a value, just like cheese. It's impossible to reach such a conclusion with a reliance on god.