Looks like the author updated the article to address that: >Update: On a more detailed examination of those two states, I’m convinced the contrast here is due

Looks like the author updated the article to address that: >Update: On a more detailed examination of those two states, I’m convinced the contrast here is due to differences in the sizes of the blocks. North Dakota’s blocks are more consistently small (StDev of 3.3) while South Dakota’s are more varied in area (StDev of 9.28). West of the Missouri River, South Dakota’s blocks are substantially larger than those in ND, so a single inhabitant can appear to take up more space. Between the states, this provides a good lesson in how changing the size and shape of a geographic unit can alter perceptions of the landscape.