I think the parable of the Magic City is fascinating because it presents us with the opportunity to examine the concept of illusions. The Magic City was an illusion for the travelers, a necessary illusion, which when its value had been realized it ceased to exist. So in that case it becomes substantial and then ceases to exist. When we think about what we learn later in Chapter XVI about the Buddha not really dying and not really leaving the world we have the illusion being the thing we no longer see. Hopefully that makes some sense. Let me state it again: The Buddha not being in this world is the illusion, the reality is that he is always present; Buddhahood is always present, though it seems to have disappeared, so the absence is the illusion.