I'm just going from what Obsidian said:
Brennecke describes Grounded as a Metroidvania-like that it'll require you to craft items in order to proceed through the storyline and the gameworld, which will allegedly be "huge," despite being made by a team of only 13. Brennecke doesn't see the team getting any bigger. "I like it small and tight," he says before mentioning that most of the people working on it wear a variety of hats.
Adam Brennecke is the game director of this game.
And another tweet from Greenburg, who works for Microsoft
If you have any other information I would love to see the source.
But yea, those other 100+ developers are working on another project.