I am sure he is right, the man knows his stuff about other peoples engines

This one

i would believe Super Mario Odyssey was built on UE4 if they said so. although to be fair, they probably didnt

If Nintendo's going to be using Unreal Engine, it's likely we've already seen footage of a game running on Unreal Engine.

Because developing a game engine is expensive and time-consuming. If it's a general purpose engine, it's a huge undertaking, and if it's not, you need to develop several engines, which is also a pretty big deal.

I'm pretty sure you don't understand how game engines work. Unreal Engine and Unity are extremely versatile game engines that fit for most kinds of games, and using an existing engine allows devs to focus on more interesting parts of game development. Developing a game engine has never been a small undertaking, and it's got much more expensive and time-consuming than ever before. Basically all time spent on developing a game engine is time not spent on other parts of a game, so it's either going to result in a worse game or extended development time. In some cases, developing your own engine is beneficial or almost necessary for a game, but in most cases, it's most likely a waste of time.

That doesn't sound like Miyamoto...

UE4 and Unity are extraordinarily versatile.  I mean, Final Fantasy VII and Dragon Quest XI both use UE4 and their art styles and aesthetics are radically different.  And then you have Ori and the Blind Forest, Freedom Planet 2, Yooka-Laylee, and Gloria Victis all using Unity.  In the hands of skilled developers, Unity and UE4 can look almost however you like, they simply provide a ready-to-go foundation.

I have no doubt Nintendo will still do plenty of work with proprietary engines.  Breath of the Wild and Xenoblade 2 are both using engines we pretty much know to be proprietary ans is Splatoon 2 from what we can see.  

There are many reasons you want to be skilled with middleware engines as a first party company.  One is use in development.  The other is assisting in porting to your machine.   

Ew, but great for rollout, I guess.

He says that the skills can be compared to the western developers. If so, then make a game like Mass Effect or GTA then.

Whatever gets the job done I guess, its only the results that matter. The games probably won't look like typical unreal engine games because Nintendo doesn't design its games the same way.

Considering Nintendo's position with 3rd parties, this was the first thing that popped into my mind.  I could see Nintendo offering help on that front to entice specific companies to port over titles that they believe to be critical towards the success of the Switch.