To me, too story-based or plot-based means that you are asking a question where different characters might arrive at different solutions and the best solution will depend on which character. For example:
Being married is inconvenient for [reasons]. Should I
Murder my spouse?
Just ignore that I'm married and proceed as if single?
Get a divorce?
Try and work things out?
The first generally only makes sense for murdering psychopaths. The second is frowned upon in polite society and may lead to the third option (or the first). Choosing between the last two depends on the exact circumstances. So we say that these kinds of decisions would be too story-based. Maybe your character is a murdering psychopath for example.
This is off-topic because any of the four answers could be correct for that specific character. Further, the character's choice may help define that character. If you want your character to come off as cold and uncaring, the second option might be great. But that's a question of what story you want to tell, not a characteristic of the world.
We can however find a similar question that is on-topic. For example, "In a world where murder and divorce are equally legal, how would that change the prevalence of each choice?" Or the same question for equally illegal. Assume laws are otherwise the same as [insert country here].
This is on-topic because it is about a characteristic of the world, not the specific decision made by a specific character. In aggregate, decisions are more predictable than in specific. Your specific character may still pick whichever option (except divorce if illegal). We helped you define the world in which the option was made.
There may of course be other reasons why such a question might be problematic. For example, the question could be Too Broad or Unclear What You're Asking.
The biggest problem that I've seen recently is that questions that are only cosmetically story-based are receiving close votes. These are questions that can be trivially rewritten not to be story-based by focusing on the more general case. There's an argument for closing these questions, but it requires an explanation of how the question can be fixed. Without that explanation, closing such questions is simply frustrating, not helpful.
I see far more questions like that than questions that are actually story-based in their fundamental nature.
In your open example, I think that the fundamental question is about the nature of the world not the nature of the character (Santa). We could trivially generalize this as a character with certain abilities. It's simply easier to describe this character as Santa, as that already gives us a description of the abilities.
See also the meta post you referenced, which specifically lists as on-topic:
Creating significant figures such as leaders, gods in a pantheon, historical figures, etc where the question relates to building of the world.
Santa would count as a significant figure. He is himself a characteristic of the world, not a mere character that appears in it.
Contrast this with the closed example. There, the question is what will these specific characters do in this specific plot situation. The answers are limitless. They can band together to kill the rebel. They can agonize but choose mercy. They can succumb to the rebel's skills of persuasion. They can do all of these things in sequence. There's no technical question, not even of psychology. Any of those scenarios can be written realistically. The author needs to choose which best fits the story.
