An author inventing a character can create any character they want. But if you're going to make a remark about a hypothetical person who could exist, don't bound the gender. The two are completely incomparable.
We could keep going down this rabbit hole. Up to you.
The joke described a hypothetical person who was born and died on certain dates, and then fleshed that out with additional invented details: gender and use of diapers at both the beginning and end of his life. If you're going to complain about the author of the joke assigning their fictional character a gender you might as well also complain about them assigning him to use adult diapers, since many real actual people would also be deeply offended by such a mis-diapering. If you're going to complain about the author of a short humorous anecdote assigning their character a gender you might as well also complain about the author of a novel assigning their character a gender.
I'm happy to have a serious discussion here but I'm not going to bother if you continue to try to dismiss my arguments without engaging.
Your argument was not dismissed, it was deliberately engaged. The person could exist with a nontrivial degree of probability, and therefore, gendering the person is inappropriate.
Also you're ignoring the mis-diapering issue.
The person described could exist, so gendering them is inappropriate if we're to fix our issues with inclusion in tech. If you're asserting that the person described is definitely fictional, sure, give them any gender. But they're not, they're a hypothetical person with non-trivial likelihood.
Characters in novels have a trivial/negligible likelihood of existing because the specificity of events described in the story rules out their existence to a large degree. Whereas here, all that's described is a potential person born today and dies 12-12-2121. Of the ~360,000 people born today, you're looking at a sizeable likelihood that someone will live that long and pass on that day.
So, since the person described is somewhat likely to actually exist, do our own culture of inclusion a favor and don't presume gender.
How hard can this be?
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Edit:
I feel myself becoming frustrated at my inability to convey the point, so I'll opt out. I regret I couldn't bring you around.
I regret not using “their,” so the reader could infer whatever gender they want, but I don’t think your argument that this isn’t a fictional person carries weight.
I'm skeptical that the specificity of events in the story matters much. For example, if I were to write a fictionalized novel about the 2016 US presidential election involving killer robot dinosaurs, would the specificity of the events involving killer robot dinosaurs mean that none of the people described in the book "exist" in the sense of corresponding to real-world people?
If I were to misgender a minor character named Caitlyn Jenner would that be ok because it's all fictional? There isn't anybody named Caitlyn Jenner who actually exists in a world with killer robot dinosaurs, after all.
This is obviously a contrived example, but we run into the problem in historical fiction all the time. A historical fiction novel might describe the main character visiting a friend and interacting with a servant, say. The main character and their friend might both clearly correspond to known historical figures, and the visit might be a real event recorded in the history books, but the servant is probably a construction of the author's mind, since servants generally don't get much space in recorded histories. If the servant is not described in too much detail, there is a high probability that the description corresponds to a real person. Is it inappropriate to gender the servant? Does it matter how closely the novel hews to what really happened?
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Just to be clear, if the author of the joke decides not to gender the subject of his joke that's perfectly fine, quite possibly preferable. But I think your position--that gendering the subject of the joke is inappropriate and should be corrected--is untenable when you start interrogating what the principled basis for that position might be and what implications there are for other situations (for example, characters in novels).
Doing that interrogation is not a straw man argument. It's the principled way to investigate ethical questions.
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I see you've already decided not to continue the discussion, so I guess we'll leave it there.