Thats ok. You just said that you can take any developer, put 800 of them in a room, teach them how to use the Unreal engine, give them millions of dollars and they will make a game as great as Mass Effect and GTA every time. No skills required.

Im doing a bit of scratching myself.

This is probably more for Nintendo devs to continue learning how to utilize such engines. Doesn't necessarily mean they'll trend towards Unreal from here on out. BotW isn't on Unreal and I don't believe Mario Odyssey is either (more in the line of an enhanced 3D World/Mario Kart 8 engine). And games like Splatoon, Xenoblade 2, FE Warriors, etc., don't appear to be as well. They'll still probably use more in house engines for big games but with the knowledge of Unreal and Unity, they could help port or create smaller budget titles faster.

Interesting, could be used for Metroid which is somewhat being hinted at..

No.  They're saying that the budget and staff size is a big part of the development of those specific games.  Can Nintendo spend the money to hire enough voice actors for a GTA game or pay for the licensed soundtrack?  If it took a staff of 800 people x amount of man-hours to make GTA, how long would it take a staff of 80 from Nintendo to make the same game?   Miyamoto is saying that they've learned to optimise the engine for their games to maintain good framerates and resolutions while still using the engine's advanced techniques, not that a staff of under 100 people is going to start churning out GTA clones that match up with GTA5 exactly.  And that is a good thing for a company that has relatively little experience using other people's engines.

From what I gathered from the article is that video games will be easier for video games built with Unreal or Unity to be ported over and not that Nintendo themselves would necessarily be using those to build games.

Yeah, because BOTW is made by 80 people...

BOTW was made by 300, true.  It was the largest development team Nintendo has ever put together, and MonoliftSoft also pitched in to help with development.  GTA5, by comparison, had a staff of over 1,000.  If Nintendo had a team of 1,000 it could have had many more NPCs, more quests, and have been out in 2015.  Now the game may not have suffered from the smaller staff, which speaks to their talent, but there is no doubt things would have been diferent if the team was 3 times as large and had a budget of well over $100 million.

actualy this sounds very well for me... if Nintendo uses UE for some of their games than great, it means faster development time.
Here is the catch.Nintendo doesn't need UE for all their games, they can use UE for some games and than work on internal engines for more complex games whit some unique mechanics.

Honestly I don't think any of their games will ever make use of Unreal Engine, they'll probably just use it as a source for what physics to apply in the game or other stuff, otherwise I think they'll have optimization issues

nothing wrong with using the same engine many other games use if you have your own ideas.