Not the centuries specifically, more the fact that the whole region has always been, and still is, a melting pot of people from all surrounding regions.

A lot of the older generation are trying to emphasize our differences and even make wild claims that I won't repeat here. Those borders were drawn by people who are very similar.

They probably mean the part about Croats and Serbs being genetically almost the same. But this is also not something surprising to me.