Because the entire point was that the character in question is genderless and this was the early 80s and also French so more modern gender neutral terms didn’t exist yet, and “let’s just smash the two gendered endings together” was his attempt at one (I’m guessing emperoratrix comes from a literal translation from French, where a female emperor is an imperatrice, and -trice is -trix in english, so imperatorice -> emperoratrix) The book also uses s/he as a pronoun instead of they.

I mean hey, it’s much more gender neutral than just defaulting to the masculine like say Le Guin did in left hand of darkness