I might be wrong but wasn't LaVey a theist in private? As in, his Church of Satan waffle that he sold to the public was an atheistic ideology, but he himself in his own life was a theist.
Anyways, to answer your question, you can be an atheist while practising magic. Think of it as the Star Wars universe (I hate to use pop culture analogies but in this case it's right). There are no gods or deities, but jedis and siths are capable of wielding The Force. The Force being a psychic energy that permeates life and can be wielded for power good or bad, but just because this magic psychic energy exists in the Star Wars universe doesn't mean there are gods. Atheist magic is the same. There's a psychic energy beneath the surface of the material world that can be tapped into and used. All the deities and stuff that people do rituals to are simply symbolic, and these symbols have a powerful effect on your psyche, which in turn produces magic. Basically "It's all just psychology bro" but has a real world impact.
I don't really belong to that line of thinking. I do believe a lot of magic is the psychic energy (ether) we tap into and that a lot of it is psychologically powered, I do think that many occultists, at times, overestimate the influence of deities and spirits instead of the true power of their own mind. At the same time I'm not an atheist, I do believe in intelligences that are far more ancient and powerful than humans and exist in other planes of reality.