One thing I want to stress again:

Yes, the population numbers are low,
But not for the point in time!

Yes, Europe had population densities of 40-100 people per square mile at the cusp of the medieval to renaissance era. (And didn't massively grow to what we have today until the medical revolution of the post-industrial modern age.)
But before then, the population density was much lower, down to that 10 people per square mile around the start of the Dark Ages. It took time for the population to rebound. And again after the Black Plague ran its course.
But that didn't mean the empty areas in Europe were suddenly filled with millions of uncounted barbarians or the like. They are accounted for in those general summary numbers I was using, even if they aren't in the LGG. And while Greyhawk does have the monsters that the medieval mind created out of fear and fancy, the critical point is they should not be allowed to overwhelm the human population density, even if that means still leaving the land below what it could support at maximum levels.

So while those numbers are low, they just represent a severely depleted initial group to build from, not some horrific monster horde swarming on everyone. Indeed, as I noted, the population is pretty much on the high end of the growth rates Europe had. Greyhawk hasn't had any Black Plague, or massive war, or anything similar causing the kind of "Malthusian Correction" Europe had gone through at the beginning, middle, and end of the Middle Ages period. Thye just had the Twin Cataclysms and the limited number of migrating Oeridians at the start. After that, they've been doing just fine, and even better than fine for the most part.