This kind of thing is surprisingly common: languages and cultures may change drastically over time, but the DNA of people living in a region tends to change much more slowly. If the DNA is indigenous, we often have to look at Y DNA and mitochondrial DNA to find the "invaders" throughout history, who may be more associated with the modern culture of that region than the indigenous people.

DNA is also very geographic: geographic distance is very strongly correlated with genetic distance, more so than cultural distance, to the point that someone reconstructed a map of Europe using PCA on the genetic code. (https://commons.m.wikimedia.org/wiki/File:Genetic_PCA_of_Eur.... I think the PCA fit is even better if you exclude Turkey but I can't find the image.)

The Americas after Columbus are an exception to this rule: native populations in most areas were decimated.