The implication is that the higher the population of sentient beings on the planet, the lower the average intelligence will be among any large sample set. This does not preclude beings of higher intellect, but it requires that on average, there is one person as far below average as the former being is above average.
Person 2: "Higginbotham's Constant in action, my friend..."
Person 1: "I was on jury duty the other day. Have you seen what the trash of society wears into a courtroom? How has society lost so much respect for anyone and anything else in just a few decades?"
Person 2: "Higginbotham strikes again..."