Liars will always use real details to cover their falsehood​s. Remember that right after it's showing, the big belief was that donk City was a hub for the rest of the game, so if Mario was the center piece, the hub theme would make logical sense for e3.
It's still not a given. There's no reason the theme couldn't have been a variety of Odyssey's worlds. To focus on one area is not completely intuitive.