Atheism does not require omniscience, or near omniscience. That is a lie that is often repeated.
Atheism simply requires a lack of belief. Atheists can, and often do, admit that they could be mistaken about the existence of God. Atheism isn't about knowing that there are no gods, but simply not believing that there are any.
And atheists, such as myself, will often call ourselves agnostic-atheists. I have said this many times at CF, and I've seen plenty of other atheists say the same about themselves. Since atheism is about a lack of belief in divine beings, and agnosticism is about a lack of knowledge in divine beings, there is no conflict between the two. Both labels fit.
Likewise, I am "agnostic" regarding the existence of elves, and "atheist" regarding them as well. I don't know that elves don't exist, but I don't hold any belief in their existence either.
eudaimonia,
Mark