Kojima says in that video that it's open world, did they just mistranslate and he just meant non-linear?
I think it's just an error in genre definitions I suppose, which are always a little dicey. From what we've seen it's definitely... open, in that you can pretty much do whatever you want I would imagine, but as far as I'm aware every mission is selected from the main menu and not picked up amidst your travels within the world itself.
...Eh, overall I think it's safe to label it as an open-world game regardless.
Ground Zeroes is a smaller scale, more contained version of the ideas they're working with in The Phantom Pain. TPP will have ACTUAL open areas to go around and do stuff in, whereas Ground Zeroes is like a small slice of one of those areas to show off the new gameplay and stuff.