I can believe that Nintendo could have a single OS, which will work on their next console and handheld.
The technology exists which would allow for Nintendo to make a handheld that is a scaled down version of the console or Nintendo could just optimize their software to work with 2 separate sets of specs.
It's possible that attempting a port could give Ubisoft an idea of that system's potential, if it runs the game well then it may bode well for ports of other Assassin's Creed or Ubisoft games in general being put on Nintendo's new platform(s).
As far as I'm aware 4chan leaked some artwork from Horizon: Zero Dawn around the time Shinobi602 started to talk about the game, so I wouldn't be quick to question the legitimacy of their ability to have any "insider" knowledge or sources that could know anything.
Sony kept quiet about Horizon, with only Guerrilla stating that they were making a new IP back in late 2010 and the odd statement about the game progressing, but nothing about it's content or the setting or lead character being revealed before Shinobi talked about it, then at E3 this year the concept art 4chan showed was incredibly close to what Horizon really looks like and is about.
This NX "rumor" or information doesn't sound unbelievable.
Maybe you can question whether Ubisoft has any faith in Nintendo, because Liberation isn't some fresh new AC game or that they aren't talking about porting a tonne of past Ubisoft games to the platform, but I wouldn't be so quick to question whether Ubisoft would work with Nintendo or if Nintendo would really make a system or family of systems like this.
