Gaiman is a fantasy and science fiction author. Some of his well-known works include The Sandman comics, and the novels Coraline, The Graveyard Book, and American Gods.
He's a bit elliptic in the post, but apparently, when writing the novel, American Gods, he made something up. Authors often do that, especially in works of fiction. Then someone wrote a reference book on something, and borrowing largely from his novel, wrote about this thing Gaiman made up as fact.
And now the Wikipedia article on this something from the novel, which used to correctly cite Gaiman as the source, now no longer mentions Gaiman at all. It now treats this made up thing as fact, and cites the reference work.
Gaiman knows how Wikipedia works, and knows he can't just 'fix it.' Because in the world of Wikipedia - "Verifiability" is more important than "Truth." And "Original Research" is frowned upon.
The threshold for inclusion in Wikipedia is verifiability, not truth—whether readers can check that material added to Wikipedia has already been published by a reliable source, not whether editors think it is true.
Wikipedia does not publish original research. The term "original research" refers to material—such as facts, allegations, ideas, and stories—not already published by reliable sources. It also refers to any analysis or synthesis by Wikipedians of published material, where the analysis or synthesis advances a position not advanced by the sources. - (emphasis theirs - follow the links above for more details)There's already a source that says this something is fact. He realizes he needs to create a source that says it's fiction - his fiction. So - he says...He could create a blog post that explains how he made this thing up, and then cite that. He wonders if he should.
He somewhat likes the notion, in a twisted-author-sense I think I understand, that something he made up is now considered a fact.
However, he doesn't progress his problem one step further. What happens on Wikipedia when he creates his explanatory blog entry, and then fixes the Wikipedia entry, saying he made this something up, and then cites his own blog entry as evidence?
Does anyone with experience editing Wikipedia believe they'll let him get away with that? Yes, Gaiman says it's the truth. But that's irrelevant. In the world of verifiability the "reference work" is probably more reliable than he is. [Note, Gaiman doesn't provide the nature of the reference work, so this is an assumption.]
Gaiman can write his blog post, but until some other equally reliable reference cites this something as his creation, only then will there be a way to 'verify' that he is the creator.
[He could also try to get the author/publisher of the reference work to publish a corrected edition, and that I suspect would be sufficient.]
A good genealogist would likely not take Neil Gaiman's word for it either. But, when he points out that American Gods has an earlier publication date than the reference work, it would suggest the need for further research. We could, for example, search Google Books for an earlier reference, and without finding one, grant that he is likely correct that he invented it. But this is 'original research.' Something genealogists do a lot of, and Wikipedians aren't supposed to do.
Update Neil Gaiman has found several more references to his made up fact in his own Google Books search, along with many other online references. So he's not up against only one source.