If that's true then...that means that XY probably has had contest stats in it's coding, or there'll be no way that'll be possible. Somehow this was overlooked...
I guess during Gen 3 and 4, contest stats became a standard part of a Pokémon's data structure. I don't think them still being present in X&Y is because someone overlooked something, they revise and rearrange the data structure each generation, so if the contest stats are still there, it can only be intentional on their part. I guess Feebas using them to evolve was a big reason for this.
You've got to admire Game Freak's efforts to keep evolutions consistent :) .