Personally, I fail to see the difference. Existential nihilists believe that life is without objective meaning. For me, only God or a Higher Power lends objective meaning to life. Objective meaning can only come from a Power greater than myself. Without God, I could certainly find meaning in the things that I do but I would have to concede that this meaning is purely subjective.
Atheists believe that there is no god or gods. From this, I must conclude that objective meaning is absent from their belief system. If not, I am certainly at a loss to understand how they rationalize this objectivism.