Nintendo has been known to announce games months before release, but they're always teasing a new Zelda.
I think part of the puzzle for Nintendo has been what to do with a title that a) clearly won't save the Wii U (as 3D World, Mario Kart, and Smash hadn't), and b) wont sell particularly well due to the Wii Us install base. For all we know it could have been on-track for its Q4 2015 release date but was delayed to release with the NX.
It sucks for fans, but it's not like Nintendo can just pull great games out of their hat for a new console launch ... having a new Zelda at release is significant. Nintendo is in the mist of an existential conundrum rn, and I'm fine with whatever they think they need to do to not go the way of Sega.
With Playtonic its simpler -- they just need to focus on making the best game are able to. Oct 2016 always seemed ambitious, but it's clear they're treating it as a serious goal, at least.
Oh, and I'm not convinced an NX port would be *that* much more work, considering they've already got builds to all these different specs (Wii U - PS4 - Windows/Mac) ... if they decide to do it, it'll be easy-peasy, right?