Which may have been tailored for the demo - for example, put a very small battery pack in there, so that the weight is much lower than the weight of the actual car would be - it only needs to drive a few meters, doesn't it?

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.