Most atheists won't say flat-out, there is no god. Because you can't prove the non-existence of something.

However, we think god is *extremely* unlikely to exist, and that there is no good evidence in favor of god existing.

In other words, the burden of proof is on people who say god exists.

So I still think it's probably an atheist bus.

They also might have picked that wording to appeal specifically to agnostics.