My guess is that this is the Great Labyrinth of Bahamut.

When I saw that part of the video, my immediate thought was that if the party was sent to the same location in the future, then they're still in Cartenau Flats, close to where Dalamud "impacted" and Bahamut was released. The way I see it, despite the sealing ritual being unable to materialize a new Dalamud around Bahamut to cast it in space, it's not impossible that it still partially succeeded and managed to at least seal Bahamut in place, on Hydaelyn's surface. It would certainly explain how the world was able to rebuild itself in as little as 5 years (with the "limited" damage Bahamut caused before being trapped again), something that would not have been possible if Bahamut continued on rampaging.