I would stay very cautious about this and not get too excited.
EVEN if it's true, it's important to note that they probably have a very different definition of "seamless" than what you could expect. When we think of seamless open world, we think of stuff like Ubisoft, Rockstar, typical giant modern AAA open world with no barriers anywhere. This won't be the case in Rebirth,
There will be places in the world with clearly defined paths, invisible walls etc, like here for example from the preview demo:
This means that their definition of "seamless" could be something like the recent God Of War for example, you can go from one point to another without loading transition, but it's still linear for a big part of it.
I'm not saying this is bad, I'm just saying people might be disappointed if they expect something different.
This also raises the question of the different traversal methods. We see flying chocobo, swiming, we can expect airship too... But how will this work when you have areas like the one I just posted as an example, with invisible walls everywhere? Most of the environment outside of that specific path are most likely not made to be seen from different angle, maybe not even solid, and definitely not meant to be explorable, which means traversal will need a lot of restriction too, and if that translates to random invisible walls in the air when you are flying an airship, that kind of sucks.