This is true. However, nothing prevents God from being "a region of space where the gravity prevents light from escaping".
IOW, your answer to the question, "How do we know that it is not God?" is that we don't know, and I agree with it.
My point is that for the lack of knowledge, we "choose this as the most likely reason out of all the other possibilities." (My emphasis.)
I think that there is no reason for this choice, but I know why I make it. I just don't like the alternative, it stinks.