So this is something of a misconception and a misunderstanding of what the study about "Genghis Khan's descendents" tells us (not to mention the uncomfortable aspects of the implied how which the tidbit skips over in pop culture recounting). Simply put, the study in question merely states that there is a common ancestor for male-line descent that correlates to around the time and place Genghis Khan lived. I've explained it in more detail here, but while Genghis is the pop culture answer to the "who", it more likely would have been a male ancestor of his a few generations previously. More importantly, the study only looks at male-line descent - son of a son of a son of a son. The number of descendents is actually far in excess of that when you account for the fact roughly half of children aren't male. Finally though, this isn't that exceptional, really. Common descent actually is, well, pretty common. Studies of European populations point to a similar confluence of common ancestry within only a little more than a millenium, for instance, and likewise while we can't actually put a name on it, Charlemagne is usually the name that is floated as the "who" in pop culture understanding, due to his prominence in the period and the wide geographic spread of his descendents over the next few generations. I've written more about this here.