"Atheism is the absence of belief in the existence of gods."
What is implicit in this definition of the meaning of the concept of atheism is that an atheist believes in no gods... It's not as though an atheist does not form an idea in their mind as to what the concept of god means to them, they do......but rather than believe this idea to be true, the believe it is not true...
Furthermore, this idea of god that an atheist believes not to exist, may not be, and most likely isn't, the same idea of god as held by other atheists, and for that matter among theists...and definitely among non-dualists...