I can't speak for anyone else, but you've made some incredible LEAPS with the your presumptions as to what an atheist 'knows' or understands(as opposed to believes) to begin with.
But peep it. If you REMOVE your assumptions about what MUST be the source of existence, and consider that 'GOD' is a concept created by man. And that existence itself, the concept of being alive, is so mind boggling complex, to think or believe that YOU or ANY being can grasp ANY insight, significant enough to describe and know ANYTHING that is so BEYOND anyones capacity to understand completely enough to point toward a single anthropomorphized entity as the source 'of it all' provokes incredulity.
The truth is we DON'T KNOW FUCK ALL ABOUT THE NATURE OR SUBSTANCE OF EXISTENCE . . . let alone a supposed single source of existence's existence, or non-existence for that matter.
So contrary to your supposition, for myself, I don't even understand the concept of GOD. Really, what IS THAT? It has EVERYTHING to do with what I DON'T KNOW more than what I THINK I KNOW. And THUS far, there's no evidence that existence requires a 'god'.
Folks get awestruck at reality, and forget that we are born into societies ALREADY culturally couched in systems of faith. THEN obliviously point at concepts born of that inculcation, not realizing that you're perception has already been shaped by the time you have the skills and level of inquiry to ponder the 'purpose and origin of existence/life' question.
Think about it.
JG