So it is obviously meant to be an absurd example. But it is that just because it is not expected in D&D, nor is there mechanic for handling it. But if we were playing a game where the players could introduce beneficial setting elements, coincidences and plot twists by paying plot points, and the player paid 12 plot points they had saved over several sessions for "major deus ex machina," then Little Boy, the ancient fission dragon swooping in to help would be fair and a perfectly legit gameplay.
None of the games I'm familiar with work like this. The purpose of players adding setting elements isn't to provide mechanisms for 'defeating encounters' or the like. That kind of terminology or approach wouldn't even make any sense. The purpose is to enrich and develop the setting and therefore the story that unfolds.
If such a game did feature a big powerful dragon, it would be something like the Odin example, or how a warlock pact ought to play out. Servicing the relationship with the powerful entity would be a big part of play, as would the cost (not financial or metagame points) of bringing it to bear.
But again we have gone here from 'I know a blacksmith' or 'This bar has music' to 'I know an all-powerful third party that will solve all my problems'. No-one is advocating for all-powerful dragon servants.But D&D does not have any sort of system for this, and isn't really built around players using setting element introduction for problem solving. So then when we go beyond what is mostly flavour, it gets somewhat tricky. What might be perfectly fine for one person might be bad faith play for another.