They still had to engineer the demo unit.
Based on the record of Musk & co straight out LYING in such events (remember Solarcity?), I am very confident in saying that a large portion of what they demonstrated they have not actually solved how to fit in a real production car.
They could be in the unit when its happening and there would still be some counterclaim.
It's amazing how on HN, a place where hackers, dreamers, builders come together to talk can't say one positive thing from some of these actual delivered engineering feats. It's bias happening real-time.