The god you describe is as relevant as Russle's Teapot. It doesn't affect us in any way, it doesn't interact with us, and apart from mathemathical curiosity there is no reason to care about him/her/it.
Atheism usually relates more to the idea of God(and gods) that actually have some relevance to human life.
For the sake of slashdot here is a car analogy:
There exists a sage who can built a perpetual motion machine(car), but he lives in a cave and won't get involved in matters of the world or reveal his knowledge to anybody. Moreover we can never reach him, contact him or even prove his existence. An Atheist in this case is someone who doesn't care about that sage, but who believes that there are no other such sages walking among the mechanical engineers judging their designs in one way or another.