Believing in the existence of a specific god and not worshipping it still excludes atheism, I would say. Atheism isn't synonymous with the absence of religious practice.

As for the thing about recognizing a supernaturally powerful being as a god, well I always thought that was an interesting question. I guess in a universe where these things are established fact, the position "but they're not gods" is the equivalent to atheism... but it's different. I think the question just moves to a different level where the concept of atheism isn't relevant.

The Athar in Planescape had pretty much that position.