Let's start with the basic essence of how closure works. One of the reasons we have closure is because a question cannot be reasonably answered in its current state. In some cases this is because the question is poorly written, but for this particular case it's because the proper information isn't distributed in the proper channels, if it is distributed at all. That's why we have the unreleased content close reason - it's unreasonable to expect us to properly answer these kinds of questions because they exist outside the scope of what our actual expertise deals with.
Let me quote you explicitly from a comment on the only answer, which states that it's unknown:
I'll mark this as accepted for now, since there's no way to figure out how to get it, but when they do announce it, do you mind updating your answer?
This statement I bolded is everything saying this question shouldn't have been asked yet. And you yourself acknowledge this. It trumps the matter of "it's in the game files" because you, me, everyone, we all know it isn't out yet. Traces of knowledge of the existence of not just Diancie but all the event legendaries has been present since December, if you knew where to look. We're not the place to look for this, though.
There's no benefit to the community to keep the question around right now. Diancie still isn't publicly released since the time the question was asked, and so far people peg it at July. The important thing is that July isn't right now, and it's not our job to be a news site to keep people up to date on things, which is the only thing that having the question around can serve right now.