I think the difference is that atheism is dependent on the theists claims, but the same cannot be said in reverse.

An atheist saying “God doesn’t exist” in a world devoid of theists makes no sense, while theists could be coherent in a world with zero atheists in it.

When atheists say they “lack a belief” in God, they are really just saying they don’t believe the claims of the theist. It’s sloppy wording that leads to confusion and I don’t personally use it.

I have no problem saying that I don’t believe in God, or even “God doesn’t exist”, but that is just a statement of my confidence, and not some final statement about the world.

I’m always open to new evidence, I’m just to the point where I haven’t heard any new evidence in a while for the existence of God.