That's so not true.
Atheism is a belief system, it is not absence of belief but belief in absence of God(s).
More accurately, atheism is the rejection of theistic claims of a god or gods existence. That's it. Period. No belief system required, no holy books needed, no ideology or dogma.
It's true that many atheists share common ground on a number of issues, but it's wrong to call atheism a belief system. It's simply the rejection of theistic claims regarding god, nothing more.
The fact that most atheists agree on underlying issues has nothing to do with their atheism although it can often be a consequence of it.