I think your logic depends on your definition of atheist. Taking the word apart it simply means without a theology. Substituting in the definition of theology w

I think your logic depends on your definition of atheist. Taking the word apart it simply means without a theology. Substituting in the definition of theology we get "without a particular system or school of religious beliefs and teachings." Under that definition it makes sense to say even someone with a monotheistic belief system is "without a particular system or school of religious beliefs and teachings" of other gods. Meaning, they don't have system of religious beliefs for other Gods besides their own.