I'm not sure it's exactly missing so much as not explicitly stated in that way. The author does partially attribute the misunderstanding to the mostly uninhabited northern territory of Canada. The author also says that Canada populates it's northern regions less than Russia does. So, the focus is more on people not being in the north than the fact that they are mostly in the south, but one conclusion casually follows from the other.

As I was reading it, I looked at the map, and concluded that a bunch of the population was likely in the Toronto area which juts well below the mean longitude of Canada's southern border.