Think about the countless numbers of ways you could overlay any sort of historical map over some map representing any kind of relevant statistic. A smaller but still countless number of them will have strong correlations by pure chance alone. This is a simple fact of statistics.
The author here is filtering out exactly those that happen to have those kinds of correlations backed by some preconceived ideas the author has about how the causality is supposed to work and uses them to fuel his hypotheses, while ignoring the uncountable number of map overlays that don't show these correlations.
This is bog standard selection bias.