I would agree they (Nintendo) decided to let Monolith Soft do a "Majoras Mask" game. They proved themselves and they've worked on a couple of Zelda games. They trust Takahashi. I don't think there ever was a new IP. I honestly think what we believed to be was a new IP was this Zelda game all along.
Because they always had their main Xenoblade team. They created a 2nd development team. Xenoblade team is fine and doing their own thing.
Let's pretend this is a mainline 3D Zelda game being done by Monolith Soft, then what's the 3D Zelda team doing at Nintendo? Are they making Aonuma's new IP that Nintendo asked him to make?
Look at this concept art Monolith Soft put up when they announced they were working on a 2nd game (the one we assumed was a new IP).
If it wasn't for us thinking this was a new IP, I could see thinking this was Zelda. The more I look at it the more I see her lol.