I suspect I'm spoiling the in-joke here, but is that a literal in-story thing, or just a fan joke? Honestly, it's getting hard to tell sometimes.

It has to deal with an author's note written after Hermione's death, in which Eliezer responded to claims that the story was insufficiently feminist with 'wait until the story is finished!', and ended with the line that she would be resurrected as an alicorn princess. It was, I believe, at the same time that the MLP community was still reacting to Twilight Sparkle being turned from a unicorn to an alicorn princess.

People have been genuinely uncertain how serious he was, given his reputation for both honesty and silliness.