OP here - please trust that I have given this a lot of thought, both through hours and hours of examining the conversation and introspecting my answers, and (months) later, in a long phone call with the director of the program, who very generously walked me through why they made the decision they did.

I alluded to this in the post - this wasn’t the only thing I did wrong, not at all, but it was by far the easiest to change and the clearest signal to the interviewers that I was overconfident and incurious, and the other stuff would just detract from the message that’s already there.