It is not specified but he uses stuff from his “open land” project. He simply explain how to use his stuff rather than really explaining how to do it from scratch. You can still download it (the link is the video description) and check what he did.
I’m still waiting for a tutorial that explain from scratch (I tried by myself, already knowing how to use virtual textures in UE 4, but with no success), no doubt somebody will come up with something soon.