lederhosen: There really should be a catchy name for the fallacy of "proof by making it happen in a sci-fi story".
llennhoff: Proof by authorial assertion?
Proof ex libris? (that's library, I know)
schizmatic: Proof ex libris? (that's library, I know). Nah, "ex libris" just means, "from [the] books," which can refer to a personal library, but can just mean "from books." So "Argumentum ex libris" would be "Argument from books." Or perhaps "Argumentum Fabulosum" would work as well. Though the best tag for what your describing would be "Argumentum ex ano auctoris tractum," which means, "Argument pulled out of the author's ass."
Context would still bet on Lakota Sioux with obsidian hatchets pwning everybody...