Because validity doesn't depend on meaning. Take the classic example: "What is north of the North Pole?". This is a valid phrasing of a question, but is meaningless without extra context about spherical geometry. The trick question in reference is similar in that its intended meaning is contained entirely in the LLM output.

I agree that it doesn't break any rules of the English language, that doesn't make it a valid question in everyday contexts though.

Ask a human that question randomly and see how they respond.