I also do not think Senior Engineers have better mental models of these code trivia questions than Junior Engineers do on average. Seniors spend more time on architecture and facilitating Juniors to be more productive so I dk.
I have never been in an interview where piecing out the problem solving and debugging outside of whatever the IDE/editor of choice was frowned upon.
Maybe they just wanted to see you solve the solution in the pair programming window and thought you were copy/pasting your solutions from someone that was helping you off screen.