You are right, the reveal that I mention was unvelied at a very recent panel, just a few weeks ago (watch the trailer, now with a modified ending; they also discuss the details later). Maybe it was a slight diversion, waiting to reveal more plot details later on, or maybe they were considering different orders of areas for the start of the game.
From what I've gathered, the attacked illithid ship will plane shift to Avernus trying to evade the chase. That's where the game will start, before the party is able to gain some control of the ship and move back to Faerūn, which I suppose will be the area near Baldur's Gate where we saw the gameplay showcase start.