Yes, but the thing you're "pre-planning" is the situation, not the story. So in the example, the dragon (its end goal and whatever steps it plans to take to achieve those goals) is the planned part. In terms of a "hook," that's all about presenting your Grim Portents to the players in such a way as to motivate them to do something about it all.
And that is very explicitly why the term "Threats" is used in Apocalypse World (and many of its successors including Dungeon World) - the situation represented by the front breaks down into one or more discrete people/groups/monsters/forces/concepts that threaten what little stability is represented by the current status quo. The Dragon is a threat because feeding its young is presumably bad for the local populace. The Magmin are a threat because they have their own agenda and want to seize control, presumably to enslave the local populace to work in the mine. The Dragon Cult is a threat because it will ultimately end up sacrificing people to the dragon. And so on.
So while the Dragon Cult might be a Threat (within the overall Front), its Grim Portents serve as the hooks to involve the players. And the whole idea of the Grim Portents is that they illustrate the escalation of the situation - first it's just rumors of a cult, or crazy rabble-rousers spouting pro-dragon madness in the market square. Then it's rampant fear in the town, followed shortly by martial law as the authorities try to root out the ring leaders. Then it's the Dragon Cult attempting to seize power. Then, once in power, it's the Dragon Cult deciding (by whatever means) who gets sacrificed to the Dragon.
At any step along the way, the players can become involved, and it is the job of the GM to give them reasons why getting involved is worth doing (i.e. one of the PCs family members has gotten implicated - perhaps mistakenly, perhaps not - in cult activities, swept up in the raids associated with the imposition of martial law, and is likely to be executed in the morning; or the cult is now in control and has decided that one of the PCs would make the perfect sacrifice to the Dragon; or whatever).
But the point of "playing to find out" is that you have no idea which way the players will jump or what they'll do. For instance, maybe if the imposition of martial law is perceived as particularly brutal, the players will actually side with the Dragon Cult. That story is just as interesting and compelling as if the players oppose the Dragon Cult from the very beginning, especially if the PCs back the cult at first because they feel the authorities are over-reacting, then realize later the danger that the cult represents.
And if the players choose not to become involved, then the Doom or Dark Fate or whatever end-condition represented by the Threat comes to pass. If the players do nothing, the Dragon Cult is in control of the human population of the island and is regularly sacrificing people to the dragon. Those that aren't already enslaved by the Magmin, that is.
Does this make sense?