He's saying that in order to judge someone's ability at something, you need to understand that thing yourself. If you require the candidate to write perfect code using nothing but a whiteboard, you need to be able to read that code using nothing but a whiteboard as well.

If the interviewer cannot do this, how is he going to judge the result? Does he know how to run a compiler? Does he know how to run the code? Does he have the skill to judge the output? Is it really a good policy for a company to discard a possibly excellent candidate that just missed something silly that would normally be checked by a tool while you type?