I don't think it's so much Tabata as it is various articles around the net like Kotaku(intentionally or unintentionally confusing people).
Notice how they(for example) say that the game is half open world, and half linear, giving off the impression that for the next half of the entire game, open world adventuring cuts off.
I don't at all believe that to be the case and I'll tell you why.
Kotaku, was quoting from Siliconera(click) and tried to sum up what was actually said. What was mentioned by Siliconera however states:
"The entire game structure for Final Fantasy XV consists of both open-world and linear parts"-- Hajime Tabata
What's key there is, 'parts.' Which is important because all he's saying is that within sections of this open world-based game, there will be a linear factor to the story. An example of what I mean is like..
Say Final Fantasy XV consists of 20 chapters from start to finish. Half may be open world adventuring all over the place, various towns, cities, and kingdoms. Basically taking you all over the world. Half of the story is linear in parts so let's say from chapter 10 and onward we play through sections of the open world in linear fashion, meaning the invasion of Altissia would be(in this example) a segment of the game comprising 5 chapters-- Or, chapters 10, 11, 12, 13, 14, and 15, all linearly take place in Altissia alone.
From chapter 15 to 20, we may be required to then travel from Altissia to Lucis, and from that point in the story, we spend the next 5 chapters there till the end. I say this because what was translated by Silc is linear parts, which is key. As opposed to him saying that the entire second half is linear all together.
I think that's where some of the confusion around the net is coming from. Read a few articles reporting what Kotaku has, and many people in the comments are a bit confused as to what this actually means because of the way in which it's being reported.
This is how I understand it anyways, hopefully it's helpful. Assuming someone else hasn't touched on this yet.