I didn't look into that, and I do agree with you.
That said, looking at the description from the Resetera post, it seems like whatever Retro Studios is working on something with "non-linear storytelling" and "World Building".
If Retro Studios is not working on Prime 4, as it's seemed to be rumored, another IP actually comes to mind from Nintendo that I can see why it's been a while for Retro, Kid Icarus.
Kid Icarus, depending on the proper hands, could be a fascinating console game, with game play focusing on open-air flight and combat would be a fascinating challenge, and that could actually explain why Retro may be taking some time on the project, due to that they have to deal with new variables that aren't from the Prime Trilogy. Not to mention, it'd be unearthing a cult-Nintendo IP, and giving it to people that could make it popular once more.