No, I agree with you. I think it works exactly the same way as other riddles, which you've just outlined. It's not that your brain is hard-wired against the possibility of a woman being a surgeon, but rather that you aren't even thinking about the existence of a mother based on the rest of the story. I mean, it's much like the, "Two coins add up to 30 cents and one of them is not a nickle" riddle.