If you were wondering how Nintendo was able to release a remaster of Metroid Prime while the fourth major entry is still in development, it turns out it was a team effort.
The credits of Metroid Prime Remastered reveal Retro Studios was assisted by Iron Galaxy - the same developer behind a number of other Switch ports such as The Elder Scrolls V: Skyrim, Diablo III and Overwatch. The American-based studio has also been heavily involved with the Xbox version of Killer Instinct.
Apart from Iron Galaxy, the remaster's credits list a number of support studios: Airship Images Limited, Atomhawk Design, CGBot, Gamesim Inc, Liquid Development, Original Force LTD, Shanghai Mineloader Digital Technology, and Zombot Studio.
While it's unclear how long this port has been in development, a number of developers credited in the remaster apparently started work at the studio in early 2019, just after the announcement Retro would be taking over the development of Metroid Prime 4. In the past few years, some of the same developers have left the studio. Retro has also been regularly recruiting.