I'm not so jaded as to think that deep language understanding isn't a useful or good thing, and I'd like to think I have some myself. However, deep language understanding is not what separates the great engineers from the good ones. In an interview, I would much rather hear a person say something about a statically declared variable with no initialization being poor code to leave behind for the next person than some…
A person who just knows it's "bad code"--but not why--is almost certainly going to leave other bad code from lack of understanding. To pull an example from the slides, the virtual destructor: making that class virtual when it shouldn't is a waste of CPU cycles _and_ bad documentation for future developers.