That statement is ambiguous between a denial of belief and a denial of the object of belief (of which the latter interpretation seems more common in ordinary conversation). If we take it as a denial of belief rather than the object of belief, then it is a mere absence of belief. The question under discussion is not over whether denial of belief is possible, but over whether it is appropriate to use the word "atheism" to refer to mere absence of belief in gods. If it is appropriate, then it is reasonable to call babies 'atheists'. If it is not appropriate--that is, if it refers to denial of belief--then the denial constitutes a belief in its own right. In that case, it would not be appropriate to call babies 'atheists', because babies lack belief one way or the other. I hope that that explains the conundrum.
It's deliberately ambiguous regards what you're stating because I agree with the definition of atheism as 'not thiesm' - ie: not believing in gods. As such, I fail to see what can be drawn beyond that from the statement.
Regards your second statement, if I put the conversationally redundant stipulation "in beings capable of understanding of a god concept' then we can exclude babies and other beings incapable of such a thing.
What you're then arguing is that I've come to some kind of conclusion because I'm capable of understanding the god concept and have. based on my understanding. rejected a belief in god, but this is too much inference. The statement ''I don't believe in a god concept I understand' doesn't explain why I don't at all. It asserts nothing about my not believing in gods except that I do not believe in gods. There's no implied truth or conviction or anything. For all you know I might understand the god concept but simply not care enough to pursue further knowledge or even thought of it. By my definition, I'd be an atheist. You may disagree with the definition all you want but that's not what I was asking with my question to Orias.
Now, I happen to think the overwhelming majority of atheists do have reasons for not believing in gods. I know I do. You could even call them beliefs. I would say it's fair to say 'most atheists have a belief about god, therefore most cases of atheism are derived from belief'. Does that make atheism a belief? No. The only thing atheism specifically talks about (by my definition) is an absence of belief in god. That's the only requirement.
Hence why I think strong atheism is atheism, but not the 'I believe Gods don't exist bit'. That is just the cause of atheism, as I said quite a while ago. If you believe Gods don't exist then naturally you don't believe they do exist, lest you be a contradiction. So the atheism may be derived, even dependent on the belief 'Gods don't exist', but it's still not a belief. I would actually suggest that the broader understanding of an atheist is 'someone who doesn't believe in God' but with the incorrect necessary inference that therefore that person believes gods do not exist. I've followed the thread a while so I know you likely disagree with me on this, but I'm only going by my experience. I think most people just think the two statements go hand in hand.
I think it's a red herring to blame political motivations in terms of a broader definition leading to an increased number of atheists - I'm sure that's the motivation for some people who'd prefer the 'absence of belief' definition. Personally, though, I get frustrated by people saying atheism is a belief because it's misrepresents me in terms of what I'm asserting about the existence of God. I'm largely agnostic about the existence of god, although I wouldn't assert my agnosticism as true - and I subscribe to the idea one can be an agnostic atheist. By people saying atheism is a belief they're trying to argue the atheist is putting forward something, asserting something about god. "If you don't believe in God then you must be A, B, or C'. This is false.