Just because the author used only confirmatory evidence doesn't mean that there isn't a correlation or a causal factor at play. The author didn't posit a theory

Just because the author used only confirmatory evidence doesn't mean that there isn't a correlation or a causal factor at play. The author didn't posit a theory; he merely pointed out that the maps are interesting. Which they are. Sure, you could probably pick arbitrary political boundaries from different times and create maps that show the opposite trend. However, I would be surprised if there weren't correlative or causal factors, given that historical people reproduced and, in the case of many agrarian peoples, didn't migrate. It's possible that our collective ignorance of history makes this appear to be a novel fact, when we shouldn't be too surprised.