Genghis Khan died in 1227, meaning ~40 generations since then. If a random person sired two sons in 1227, and each of them had two offspring in each generation who in turn had children, that's 2^40 = 1e12 progeny today.
They mean surviving progeny with his DNA. After 7 generations, most of your descendants are not actually genetically related to you.
By contrast current estimates are that about 0.5% of the world has a y-chromosome descended from a relatively close ancestor of Genghis Khan. (When his horde burst out, both he and his male relatives took advantage of "mating opportunities".)
A side note. His y-chromosome would have benefited more than his other chromosomes since many of his sons, grandsons and other direct male descendants also had good breeding opportunities. Those men had his y-chromosome, but not all of his other chromosomes.
Your very own numbers should immediately tell you that they're useless, since there aren't 1e12 people today. What's the point of comparing Genghis Khan's empirical reproductive record to a hypothetical, known-to-be-impossible, known-to-be-not-even-slightly-related-to-what-could-have-theoretically-happened reproductive record?
