I'd say this is a deepity rather than actual wisdom. Even if you could measure intelligence with enough precision to allow you to determine the one person who is the median, there would be HUGE numbers of people on either side of the median that are so close to the median in intelligence that it would make no difference at all. You would never be able to determine that they were any more or less intelligent than the median on a day-to-day, because the difference would be so tiny. So in reality there would be some significant proportion of people who for all intents and purposes are as smart as the average person, leaving far less than 50% to be less intelligence than that.
That, plus the fact that you can just as easily say there are 50% of people who are smarter than the median as well, thus making the exact opposite point using the same assumption, renders the whole thing rather meaningless. Especially in the context of game rules written in a book. Who is more likely to pick up a book to play a game from it, someone of below-average intelligence, or above-average?