If you’re wondering why this image is relevant, it’s because, if it is real, that means the game has begun development. And that’s not as expected some people might think. Masahiro Sakurai said the game hadn’t begun development a couple of months after the game was announced at E3 since he was focusing on Kid Icarus: Uprising. That might still be the case, but I wouldn’t be surprised if he’s wrapped up that work.
The game felt very polished even back when I played it in January in Japan, and again when I played it at E3, so it wouldn’t be that surprising. I think the reason the game hasn’t been released is because it would lose the spotlight to games like Super Mario 3D Land and Mario Kart 7.