> But it seems to me the lesson from this paper is that this (isolating us in separate groups) would make the split complete enough that we would decisively start butchering each other.

of course, and historically we can see that from the past 300 years leading up to ww1 and ww2; every empire was in it for themselves and very nationalistic, mercantilism ruled the day, and lots of crazy theories such as phrenology and eugenics started to appear leading to all kinds of atrocities...