Agnostics

[Excited Penguin]

That is simply inacurate. Lack of belief is an acceptable form of atheism. Lack of knowledge of existence of belief is perfectly compatible with atheism as well.

Certainly, I lack belief in God myself rather than expressing certainty in the nonexistence of any kind of God or god (I am a gnostic atheist regarding the tri-Omni version and literal Biblical version, however). Lack of knowledge of existence of belief renders atheism not even an opinion, and I think it is at least that.

But I'm aware that my opinion doesn't represent the majority here. I don't think it's splitting hairs to observe that a rock isn't a person ant that the suffix 'ist' can't apply to a rock, though. A rock does fit the definition of 'atheist' that you give though: it does lack knowledge of existence of belief.

A rock doesn't fit the description of atheists, since atheists can only be people. Atheism, in turn, refers to only people as well, but that is something you have to figure out on your own, 'cause the dictionary doesn't spell it out that only an intelligence can disbelieve something, can lack information on a subject, and so on.