First off, people disagree about which questions, exactly, are on topic and off topic respectively.
If everyone agreed, this wouldn't happen. Nor would we need for five people to vote to close before putting a question on hold; a single vote would be enough.
Sometimes, some people see a question (in this case) as asking about the world, while others see it as asking about the story.
Worldbuilding is inherently a subjective process, and our continuing discussions about what particularly "primarily opinion-based" means in the context of Worldbuilding SE is probably as good an evidence as any of this.
If you have a decent idea of the site's scope (and it'd be hard to get to well over 10k rep without that), then answering a question that you feel is on topic is fine. Voting on answers is fine, even if the question is poor or off topic, but make sure you're voting on answers based on the answer's usefulness to the question, not whether you think the question is good or bad. (The two are distinct.)
What I personally really don't like to see is when people both end up participating in closing the question, and answer that same question. If the question is good enough to be answerable, and on topic enough to fit within the site's scope, then it shouldn't be closed. While people can disagree about whether a question meets that bar or not, one would hope that any single person can reach a conclusion either way.
On the flip side, if you see a question drawing close votes but you feel it is on topic, consider editing the question to emphasize the on topic parts. Sometimes this can be as easy as deleting some extraneous material and highlighting the specific question; sometimes it takes larger edits that need to be done by the OP.
As an aside, I personally think that the question you linked to is more about an element of the world than it is about the story, but that's just my opinion. Sure, there's some story to the question, but I see that as mainly background material; the question itself seems to ask about an effect of the magic of the world on beings in the world. That said, it'd probably help the question a lot if the magic was described in a bit more detail.