| So in the end, about 50% of the world population lives in about 50% of the world's habitable land.
Your math may be accurate, but your assessment of habitable land-mass seems to be off.
I know and work with a number of people in Canada, and I've visited many, many times. I'd hardly consider the entire area of Canada "uninhabitable". (Per wikipedia [1], Canada has a total (non-water) area of 3,511,023 sq. mi.) 37 million people live Canada [1].
Also, just a quick visual check indicates that about 1/2 of the area within that circle is ocean. And, the population density of the Himalayas (which is in the circle) is arguably small as well.