At the same time I can see his point. "Understand" implies that the thing dominates you. In fact, when you fully comprehend something, you don't stand under it, you dominate it. To have full command (notice the word) of a topic, or issue, or skill. In Spanish the word dominate is used directly, "dominar". If you're an expert at something, you are said to dominate it. In English we say master. I've mastered (again, notice the word) welding, computer programming, engineering, pipe organ, etc. They didn't master me, I mastered them.

I think that's why the weirdness of saying UNDERstand bothers him and his cohort.