[Jongseong]

Linguists must be rolling their eyes at this study. If you want to study language evolution, phonemes are the last thing you should look at. Languages change, fast, and the sounds of a language change drastically over generations. Compare modern Icelandic and Old Norse (by which we usually mean Old Icelandic)—Icelandic is supposed to be one of the most conservative languages and is still quite close to Old Icelandic in spite of centuries of separation in areas like syntax and vocabulary. But if you compare the pronunciations, they are quite different. The sounds of a language are among its least stable aspects.

Similarities in pronunciation between languages spoken over the same region are part of a phenomenon well known to linguists and do not imply that the languages are related. For example in terms of a pronunciation feature called coda devoicing, Slovene dialects near the Austrian border will pattern with German, those near the Croatian border will pattern with Croatian, while Standard Slovene will have something in the middle. Slovene and German are only very distantly related.

There may be something there about closely related population groups ending up using similar sounds even if their languages are not closely related, but I doubt the researchers even thought about that and used phonemic distance as a shorthand for actual linguistic distance.