you are indeed correct, there is only one true God. However prayer is a form of worship, and worshiping anything other then God is sin, even if your worshiping a notion of a god or gods, because your not giving the Glory to God.
So in this point I'm trying to keep other from sinning, on the matter of the Prayers of nonbelievers reaching God even though it wasn't to him they prayed. Yes, God would indeed here the prayer, however God is not going to act(generally) without your direct consent.