TL;DR : No proof is needed for two reasons : The 3rd party rule is not for legal purposes, nor are we in a position to claim in the real author's stead. Of course, this doesn't protect the question from other closure reasons.
Why law is not the the key factor here?
Law doesn't happen here for many reasons, but roughly :
- Everyone can ask questions about public works as long as they are not copy-pasted in the question. This is not falling at all under the copyright of said work. And if you think about it... First and otherwise the whole Sci-fi & Fantasy stack-exchange, wikipedia and any fan sites could be shut down for that reason. Then, this would be absurd to prevent anyone to talk about the movie they just watched. It would be akin to a total destruction of freedom of expression.
- This site works almost exclusively with ideas. Ideas when creating fictional worlds are not copyrightable in most countries1. Indeed, ideas are fluttering, ethereal butterflies in the sky, coming in and going away freely 🦋. But worse, noone could prove who catched one of them first. More factually, protecting ideas prevents people expressing said ideas, rendering the law's purpose of protecting creators useless. Skipping all the details, only what you or I have written word for word is copyrighted, and as long as you don't copy-paste it as it is or make derivative works of it, you aren't even restricted by the CC BY-SA 4.0 licence Stack-Exchange uses.
Even if there was indeed a law protecting something somehow, it is not our legal responsibility to make others respect it. It is the responsibility of the intellectual property owner and Stack-Exchange to take action when illegal stuff happens like breaching NDAs or infringing copyright.
You can (ideally should) warn the content owner of potential infringements so they can take appropriate measures -like contacting SE's designated agent- if they wish so. You can warn the SE company of possible troubles, so they can take preemptive measures if that's clear and needed. You can and should also keep people from raising obvious red flags for their own sake, both to learn worldbuilding and to be friendly. But closing and/or deleting in SE's stead? It's not our job, since it's impossible to know at all whether the user is the actual owner. We're just for most anonymous pals on the internet who can only reasonably argue they're who they are by telling their "proofs" are the truth. It's a very weak argument called circular reasoning. And nobody as honest as they can be can do more than that without excruciating efforts which are not worth it.
But why does the 3rd party rule exist then?
That's the most important question here. For that I'm going to quote Sphennings, on a similar topic :
Regardless of what inspired your question, existing work of fiction,
myths you grew up with, a wild dream you had last night, the sublime
majesty of the natural world, or some cool science fact that ignited
your imagination, you must always make sure that you're asking about
that within the context of building your world.
- Don't ask about elves, ask us about your elves.
- Don't ask us about Skrulls, ask us about your Skrull knockoffs
- Don't ask us about Kami, ask us about your Kami
- Don't ask us about orbital mechanics, ask us about the orbits of your planets
That's the problem with 3rd party worlds : Most of the time, people don't ask how they can "build" something, not even how it "has been built" by other people. They ask how it "is" in one's world. This kind of question is therefore not building worlds, ie. off-topic per the tour and help-center2. Whether they claim ownership is irrelevant to whether they are building worlds or not.
Regarding the question
That being said, the question is suffering from over-contextualization. All of it could be summed up in one paragraph describing the kind of bomb, one picture showing the map of the now destroyed England3, and one ending sentence asking the question. Most of this confusion could have been avoided that way.
Moreover, there are several questions at once with hints the querent isn't solving a specific problem, but making the whole element. Indeed, you cannot describe the impact of the bomb if you don't know how the bomb works, regardless of its actual size and power. Since the question talks about an unknown, never described phlebotinium bomb, both are asked here. This is a big issue and the reason I voted to close (lacking focus).
1 : Any country who signed the Berne Convention would probably follow it one way or another, as it's the expression of the idea which is protected there.
2 : I made emphasis on the core definition of worldbuilding here
3 : Map which could gain from being more accurate. It's so tiny for my sore eyes ^^