It is possible to be both an atheist and believe in magic. Indeed, in our
own world PEI Bonewitz appears to do this. He has a completely
mechanistic metaphysic, one that explains "gods" as merely being resonant
psychic networks, not actual beings at all in their own right.
In Glorantha, one can be an atheist and still admit to the existence of
Orlanth, et al. One simply denies that they are gods.