It's not about premise, it's about we know what these guys are capable of and what they would've done with Star Fox.
Nintendo's strength has always been that their control mechanics have a reputation of being polished and head and shoulders over the wonky messes other developers put together. If you want a perfect example, compare the Namco programmed Star Fox Assault controls versus the Nintendo programmed Star Fox 64.
Sakurai isn't a development entity, just a single staff member. With that said, his games aren't exactly known for having tight controls. Treasure is incongruous to any development project that is AAA - HD. Nintendo's Sin & Punishment 2 producer Yagamami reiterated how S&P2 was a huge ordeal to get done because it's kind of above Treasure's comfort zone in dealing with schedules and big budgets.
Platinum couldn't have done the game themselves at this point with the meager staff they have allocated around projects. Secondly, Miyamoto would still only have funded that game if it was validating the GamePad either way.
Did you read the link and quote ? Back when it was unveiled as a prototype, Miyamoto expressed the wish to finish it within a year. 12 months from a prototype to gold. And they still were looking for a partner to developp the game.
How can anyone like him imagine to be able to ship the game within a year starting from nothing ?
It wasn't starting from nothing. It was a work-in-progress prototype that the in-house team worked on for the past 6 years. It's exactly how the Pikmin team and other internal teams operate. When the game goes into full production - meaning the inclusion of a full production staff - that's where PG (and Beetribe and whoever) joined in.