You mean like a legitimate, playable leak? Haven't heard of one stalling or delaying a release yet..
If the game is leaked, it doesn’t make the same smash (no pun intended) the game would make if no one knew a thing. People would die to get the game, especially if the release is close after the announcement day. If people know about the game in advance, then the smash is reduced, which means consumers reduced, which means profits reduced. Less people will hear about the game due to less media coverage, and the devs won’t make as much money, which would lead the devs to feel a need to delay the release and add more content or something unexpected, or not release the game at all.
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Exactly the fact that is on the Nintendo servers is confirmation enough, I don't know what elsehewants.
Now to be fair, there is still one other possibility, though unlikely. It was a placeholder on Nintendo’s servers, so just in case Epic Games did apply for release, Nintendo would have a spot ready. Again, that’s only like a 3% chance I’d say but still.