Atheism is the rejection of theistic claims.

Yes but there's generally a reason for that rejection. If we're talking about the "new atheism" prevalent on Reddit with the overt focus on science and empiricism as the only paths to valid knowledge then that would be the type of atheism I'm discussing here. Atheism predicated on a metaphysical worldview that has been largely debunked.

It has nothing to do with any view of the universe.

I'd say the vast majority of the atheists on Reddit fall into the metaphysical naturalist bucket. That's the positive philosophical claim they're making and the one that informs their rejection of theism. I'm simply pointing out that it has it's own flaws.