We have two general principles that apply here. They may be conflicting, which is interesting!
The first principle is that we want to tackle claims that many people believe. (We use commonly repeated, or made by a notable person as a proxy for this.) If everyone (or even everyone but the OP) understands it to be a joke, it isn't notable. However, if many people are confused by it, and believe it is true, we should be willing to tackle it.
The second principle, derived from questions about claims in movies is that we only want to deal with propositions that the "claimant" is seriously claiming. If the author doesn't expect people to believe the claim is true outside of the fictional universe they have created, we don't want to be bogged down answering it.
I figure the first principle outweighs the second. If people believe it in, despite the author never intending it to be taken seriously, it is worth addressing. Alexandria's Genesis is a good example of this.
[Full disclosure: I am strong advocate of chocolate cake, although I have never had been paid by Big Cake to promote it.]
