Assuming that this God 'believes' that nothing created it, it believes that it is the supreme truth of existence ect. ect., and that there is no higher power, would this God be considered an atheist?
God does not 'believe' there are no higher powers. He knows there are no higher powers. There is no belief or lack of belief. There is only knowledge, for God.
He knows that there is one supreme force, and that is Himself. Belief is meaningless for God, in terms of His omniscience.
No. God obviously believes in Himself. "Atheist" means not believing in God. It doesn't mean "not believing in a power greater than itself." Atheists do believe in a higher power. It's called "nature." They don't believe in a DIVINE or SUPERNATURAL higher power, however.