Grant has said a couple of times that the map is supposed to follow some Kabbalistic attributes in nature, the Kabbalah posits that God has about three aspects, or forms that it took. The Ain is the non-existence preceding God, the ain sof is God, and the ain sof aur is the infinite light of God.
Grant, I believe, did personally think of the Source as the Ain Soph Aur, the infinite light (or in this case, energy) of God. And the map does say that there is specifically three things beyond the Source wall, which Grant has called God.
Due to the Kabbalah calling these three manifestations "God", I believe Grant probably also called the Source, "the unknowable" and the Overvoid "God". Hence, why the Source was referred to as the Overvoid or the white page multiple times, it's simply synonymous with God.