It just means they haven't modeled the externalities. A plane on the ground isn't emergent. In the air it is, at least until you perfectly model weather, which

It just means they haven't modeled the externalities. A plane on the ground isn't emergent. In the air it is, at least until you perfectly model weather, which you can't do, so its behavior is emergent. But I think a plane is also a good comparison because it shows that you can manage it; we don't have to perfectly model weather to still have fairly predictable air travel.