Don't think you are right about the game being developed at Nintendo Japan.

“Specifically, the highly-anticipated Nintendo Switch exclusive is being developed by Bandai Namco Studios Singapore, which we understand is lead studio on the game in collaboration with Nintendo,” Eurogamer said. “This studio houses some former LucasArts Singapore staff who previously worked on, among other things, the ambitious cancelled Star Wars 1313.”

The article is based on unnamed sources, but also includes a good amount of circumstantial evidence that suggests the rumors were correct. The company has a close relationship with Nintendo, as Bandai Namco studios have previously worked on the Mario Kart arcade game as well as Super Smash Bros. for the 3DS and Wii U and Pokkén Tournament for Wii U and Switch.

https://www.polygon.com/2018/2/9/16994730/metroid-prime-4-developer-bandai-namco-nintendo-switch

It's all rumor of course, and I certainly stated I have no basis for my speculation.  But just because Nintendo denies something or Retro denies something doesn't mean it's true.  They're masters of expectation/rumor control.  Also, Retro continues to post pictures of prime rib on their Twitter feed.  If it's a joke, it's a sick joke...albeit, a delicious, tasty joke...But still sick.

I wouldn't necessarily say this is definitely a tease, but I think after the Cranky Kong debacle a few years back, Nintendo have a much better sense of the kind of announcements and trailers that the Game Awards' audience will expect or respond to, and Metroid fits the bill. It's a more 'serious' Nintendo title which will target an older demographic, and it's something critics and journalists will get excited about, which will move the hype cycle.

If you think, since the Cranky announcement at the Spike Awards in 2013, at the Game Awards Nintendo have shifted approach. They had early Zelda footage in 2014 (the first gameplay footage of the game), a Zelda trailer in 2016, and Bayonetta 3 and the Bayonetta ports for Switch in 2017 (I don't think Nintendo announced anything in 2015?). The emphasis on games like Zelda and Bayonetta, means that something like a Prime 4 reveal would make a lot of sense this year. I'd been hoping it would happen - we might even see a Prime Trilogy HD announcement as a release for early 2019, as with last year's Bayo 1/2 ports.