This is incorrect. If someone asks you whether or not you believe in God, there are only two suitable answers. You either do or you don't. If you don't, you don't believe in God and are therefore an atheist on the basis of that belief alone. Furthermore, that is how you're born. You don't enter the world with a belief in concepts you haven't been introduced to. If I had gone my whole life without having ever heard a single thing about the existence of giraffes, I wouldn't have concocted a belief in them at some point. I would've always had no belief. Additionally, atheism is not a 'belief system'. That would make it akin to religion. Atheism is just a response to a question. Anything you tie to it doesn't reflect upon it as a whole. There is no dogma that can be associated with it.
This is too simplistic , I think. Especially the comparison to giraffes. For example, say you're born into this world and no one even brings up religion, but you experience loss, and realize that everyone dies. Thus the mystery of life will probably enter your head; what's the "point" if we all just die? where do we go, if anywhere? Maybe you answer this by saying well there must be no point. But maybe you say there must be some meaning to all this and develop some type of religion.
Now there would be no reason a person would question whether giraffes exist with no evidence of them whatsoever, and no one bringing them up. But religion, or some deeper meaning, could easily be thought up without anyone mentioning it.