Both Gardner and letter writer Steve Brown evaluate the intelligence of the average moviegoer. There is no average moviegoer. The motion picture industry may use the idea of an average person to gauge and strengthen its marketing possibilities, but this average has only to do with the patron's capacity to spend money on the movie.
Like other art forms, an inspired patron has to wade through a lot of work that will fall short of expectation. But when the magic of a well-crafted story works, in that amazing larger-than-life arena, you know it works like nothing else.