This is a line of thought that 'atheism', though wishing to define itself as independent of God, is inevitably already defined in reference to God, and is therefore a sort of subset of religious belief. You can see why the logic appeals. But to say 'atheism is something always already grounded in theism' is to work with the presupposition that God is the ground, and the other quantity is always thrown into relief by that ground. The problem here is that the opposite argument has exactly as much legitimacy. In other words we could argue that godlessness in the ground -- let's say, the whole empty, rainy, stony Earth into which early man came into consciousness -- and religion, prophylactic against angsty nihilism and despair, is always thrown into relief against that ground. That atheism, in other words, is prior. So, not that secularism is a religion, but religion is another kind of seculairsm. That theism is another kind of atheism.
That's not to say that there's a way of deciding which of these, ying, yang, is the right way of looking at things, of course.