Atheism is lack of belief in any god existing, not disbelief in the possibility of a god existing. I don’t know where you’re getting that definition from, as I’ve never met an atheist who would agree with it? If you ask atheists if they are open to the idea of a god clearly intervening in the universe in any meaningful way, thus proving both their existence and their status as a god, I think the majority would say of course. Many of us would also be disappointed as those gods tend to be selfish, mean, exclusionary, and anti-nature in their treatment and expectations for humans, but we would accept it as reality.

It just seems unlikely to happen given that none seem to have done so yet. If you want to speak of a god in terms of a “spirit of the universe” or similar nebulous impersonal, non-intervening concept, that is hardly different from just believing in the teapot or unicorn, in that it will have zero influence on your life and you may as well not even concern yourself with beliefs at all. Which, coincidentally, is how most “believers” live their lives anyway - without regard to any of the supposed commandments their gods have instituted. It’s like they only believe in the idea of belief as a virtue unto itself, not the actual beliefs. And thank goodness they only go that far!