The only thing I can think of is they were fielding for them to say "that's not possible" and push back, potentially as a way to gauge if they would reject unreasonable client expectations or something.

Possible but risky in an interview. Some people are really unreasonable.

Then it's a stupid way to check for it, because it was possible and trivial enough to implement in a short time frame with a "rewrite" in the middle and no attempt to meassure pushing back in other ways when the first one failed