Why is waste heat a big Moon-engineering problem?

Answer: Vacuum kills easy air cooling

Vacuum kills easy air cooling ✓ — Correct! On Earth, many systems dump heat through flowing air or water. On the Moon there is essentially no atmosphere, so a reactor must reject waste heat mainly through radiators. That makes heat rejection hardware large, exposed, and central to the whole design.

Heat makes fuel stop fissioning — Not quite. Reactor physics is not that simple. Heat management matters for safe operation, but the specific Moon challenge is how to get unwanted heat out of the system once it has already been produced.

Weak gravity traps hot gas — Not quite. The main issue is not hot gas getting trapped by weak gravity. In near vacuum there is barely any gas to carry heat away in the first place, which is exactly why radiators become so important.