Atheists, by their nature, want proof before they subscribe to a position or a theory. That's why they're (we're) atheists in the first place. But the same sort of critical thinker that would normally be an atheist, is the same sort that is willing to modify their beliefs in the light of strong evidence that they hadn't heretofore considered. So the statement:
"If God appeared in front of me, I Might change my mind"
is perfectly consistent with the position that "I don't accept your fantastic theory without proof, but if you do come up with the proof, then show it to me and we'll talk."
To me it's the only tenable position of the lot.
It's not like your car example.
It's more like:
I don't believe your theory that there are flocks of purple sheep on the far side of the moon. I can't disprove it but it's not up to me to do so. I'm perfectly entitled to say there there ARE no such flocks of sheep. At the same time, since such a thing is unknowable to me, I can't say without any doubt that it is absolutely impossible. Show me evidence of the sheep, and I'll reconsider.