The numbers attributed to genghis khan and the mongols are pretty much scientifically impossible. It's propaganda/folk lore than real.
> It's estimated that 15M died in the Mongols' five-year invasion of central Asia.
Estimated based on what? Myth?
> Rome used to turn the defeated tribes into slaves (though it was not a permanent status) or tax-paying citizens.
And the mongols turned the peoples they conquered into tribute states. The mongol empire was one of the most inclusive and peaceful empires in human history. They accepted all races, all religions and all ideologies - as long as you accepted mongol rule and paid tribute. What do you think yuan china, duchy of muscow, mughal persia and a host of other khanates were?
The mongol army generally consisted of a few thousand to 30K and their campaigns were largely targeted at cities and capitals rather than populations. And keep in mind that during this time, most of the human population was rural rather than urban.
The idea that the mongols killed that many people is absurd. Not only did they generally target cities/capitals, once the ruler submitted or was deposed, the mongols generally went home and left a small contingent.
You shouldn't take anything you read in WaPo at face value. But even more, you shouldn't anything you read about the mongol empire at face value either. There is so much nonsense and myth that is written about the mongol empire.