Apparently there is a little more to the delay, according to Game Informer's senior editor Imran Khan and his sources.
https://mynintendonews.com/2019/01/25/game-informer-senior-editor-tweets-what-hes-heard-regarding-metroid-prime-4/
It looks like Nintendo initially aimed to have multiple teams from different countries work on different aspects on the game, which apparently is not an uncommon practice in AAA development at least. Then, as development was not going where Nintendo wanted it to go to meet their standards, they decided to redo everything under their own internal team under one roof with Retro Studios.
Edit: duplicate phrase in the last sentence.
Last edited by Kai_Mao - on 25 January 2019