Always more to be said but there's a decent discussion here: https://www.reddit.com/r/AskHistorians/comments/3emx17/everyone_knows_that_05_of_the_population_is/cth4osq/?context=999 between u/Georgy_K_Zhukov and u/terminus-trantor which sums up some of the difficulties of that claim, it talks only about direct male-line descent, makes some very broad generalisations from a fairly small sample set and can't be proven to actually be anything to do with Genghis Khan personally at all.

More generally, by pure exponential mathematics, it's worth pointing out that virtually anybody who lived 800 years ago and has living descendants is likely a direct ancestor for a fairly large chunk of the world's population.