They never discuss these things openly, more an assumption there:
- The Rare thing was rumored back when Microsoft acquired the studio.
- Miyamoto said in 2013 he could have seen Retro handling more projects at the same time, but we never saw that happening. They worked on Tropical Freeze, after that there was a new game in development in 2014 and then who knows what happened?
https://nintendoeverything.com/miya...very-high-priority-for-new-potential-metroid/