One of many problems with this sort of study is that the correlation between brain size and something we would measure as intelligence is not possible within a species, let alone between species.
For example, among humans, men have larger brains than women (because they have bigger heads). However they have the same average intelligence.
Also, in humans there is no one to one correspondence between brain size and intelligence.
It is interesting the sorts of things people get other people's money to investigate. The underlying reason for the research is to provide additional material support for the theory of evolution, though it doesn't actually do that. If correct and meaningful it would only provide modest support for the idea of natural selection, which is not in dispute.
It is interesting though. Similar results have been found in bats (the "brains to balls ratio") though I don't recall if that was a rural/urban distinction.
Thanks for posting.