No, God is not a theory, or more accurately it is not a hypothesis, in the scientific sense. A valid scientific theory has to be testable and falsifiable.The existence of a god is a theory. It's just never defended scientifically, because it's all about feels.
If someone thinks Atheism is anything other than the lack of a belief, then that person doesn't understand Atheism.
That is why many scientists believe religion is outside the boundaries of the scientific method.
The type of atheism being duscussed by the professor is the declarative and unequivocal rejection of the possibility of God and a spiritual higher power
That is why agnosticism is definitionally distinct from atheism