What Dawkins argues is that a real explanation explains that which is more complex in terms of that which is simpler. Explanations of anything in terms of God necessarily explains things in terms of that which is still more complex, and so such explanations are nonstarters, since they fail to explain the more complex in terms of the less complex.
The logic of this position is that evidence for God is impossible, for if there were evidence of God, it would provide us with an explanation of the more complex in terms of the less complex. But this is impossible by definition. The search for such evidence is doomed at the start.