I'm leaning toward it being seamless largely because of the boat. That doesn't necessarily mean it would be open world though, at least not to the degree X was, I think they'll still gate progression in some way tied to the story. But the boat has me daydreaming about Monolith trying to implement the classic JRPG structure of gradually opening the world as you get more vehicles - something that tends not to be done as much or as freely in 3D games as it is in top-down pixel art games.
I'm hoping we get an airship. It's been pointed out the boat looks a little like Junks. In the final third of the game, the boat could be upgraded to fly, and then the game world is fully opened up to the player. It'd be like Dragon Quest games introducing the boat about a third of the way in, and then upgrading to the flying phoenix/dragon/whale/hot air balloon toward the end.
I swear there was an interview where Takahashi said they originally wanted the Bionis to be seamless and allow the player to fly freely around it at one point (probably after Mechonis Core). I can't find the interview now, I'll need to keep searching (anyone else remember reading that?) But I think it's possible he could resurrect the idea here. Although hoping for a seamless world, an airship to fly around it, and underwater exploration/combat, maybe I should tone down my expectations somewhat.
Oh, I know, I was just teasing. Between the simius battle video with no explanation as to what's going on and the eight minutes of Gramps talking nonstop in the middle of another direct, Xenoblade's made some unusual marketing decisions.