Taking well over a minute off the world record is an amazing leap. The runners who dominate this and all distance events, are not all Kenyans, but are all from the Great East African Rift. While they train hard, very hard to reach this level, it is mostly training to beat other Kenyans, Ethiopians, and the occasional Tanzanian, Eritrean, or Sudanese. The Japanese do well, and Americans are making a resurgence under Salazar, a former marathoner who is now a coach. But Rift Valley runners still outpace everyone.
Yes, of course it's genetic, related especially to slow-twitch muscle fibers and narrow ankles on long calves. Other advantages, such as lung capacity or oxygen processing can also be genetic, so it doesn't mean that one group will always dominate. But the Kenyans dominate, as they do in other distance events, especially the 3000 meter Steeplechase. The current world-record holder, running for Qatar, was born in Kenya.