It is so difficult to write challenging exams that only top-tier (“devoted”) professors do.

That’s what made his classes brilliant. IIRC, the exams were not questions you could memorize the answers to. Instead they were problems you had to solve in real time. You had to exercise the problem solving and algorithm building you learned throughout the course and discover the solution. I never felt so smart, even now, than when I figured out the solution to a difficult problem on those exams.