He's accomplishing what some of the young french defenders his age have struggled with and that is defending without relying so much on just pace and physicality. You see it with the likes of Kurt Zouma, Mangala and even Varane that they rely so much on their athletic builds in order to bail them out defensively.
Umtiti isn't the tallest nor the strongest out of that group but he easily a much smarter defender/player out of all them by a good distance. Having strength and pace is great but relying squarely on those attributes in order to defend instead of using your Brain isn't a good idea.