That is a bad comparison IMO. We have piles and piles of hard evidence the Earth is round. Saying the Earth is flat is just factually incorrect at this point.
We also have a lot of evidence that snakes can't speak, people can't turn plain water into wine, walk on the water and so on.
But the existence of God. I would argue we have no hard evidence of God's existence nor do we have hard evidence that God doesn't exist.
Claiming something which can neither be proven or disproven is what constitutes a pseudoscience. By that logic I could claim that we are in fact giant pink elefants hopping around on the moon, while imagining our reality as we currently think to perceive it. Since you can't disprove that, I must be right. Or am I not?
As far as science is concerned it is still a theory.
No. A scientific theory can be proven or disproven, while the idea of a God, as interpreted in most religions, can not. Thereby constituting a pseudoscience. And thus, it's not a scientific theory.
On top of that what makes a god a God there are multiple definitions of a God.
I suppose in the context of the parent comment the abrahamic God is meant, as interpreted by Christians, Jews and Muslims.