Mm....
That's not necessarily true.
Brawl's roster was leaked months before it was released. Most of Brawl's stages were also leaked as well.
Similarly, several new Kalos pokemon and mega evolutions were leaked before Pokemon X and Y's release as well.
Nintendo does slip up sometimes. They're only human.
WHOA WHOA WHOA. POKEMON X AND Y, ARE YOU SERIOUS?
If you know anything about Pokemon, you know that Pokemon leaks happen so often, I'd call them goddamn inevitable--almost tradition. CoroCoro scans, early release dates abroad, mistakes in trailers, hackers... you name it buddy. Remember that GameFreak is not technically Nintendo. It is a separate entity almost, but only owned by Nintendo.
I like to think of Gamefreak like a Clownfish, acting on its own, while Nintendo, the anemone, provides it a home. BIG DIFFERENCE.
And the brawl leaks happened very far into development, near release dates. That's the difference with the Palutena leak and the real leaks during Brawl-era.
EDIT:
Nintendo, as the giant that it is, has much less room to make mistakes. GameFreak, on the otherhand, is not so stable (information-wise) and history shows that to be true.