At best it proves the concept of gods exists and I doubt anyone disagrees with that, you can't really argue that a thought can't exist. What it doesn't prove is that God exists as some material or immaterial entity and that's what atheists claim, that there is no existence of any entity that could be considered a god.

Why it doesn't prove the existence of gods is simple. If the proof is that it exists because we thought it then dragons exist, faeries exist, even flat earth exists because there are people who think it exists. I don't think I need to bring more examples to show how ridiculous the premise is. Just because we can think of a thing doesn't mean that thing now exists.