Peter Molyneux has no clue what he's talking about there. Nintendo didn't make Rare secretive, they were that way back when the company were known as Ultimate: Play The Game, which was in the 8bit days long before they became involved with Nintendo.
Part of the reason why the company was so detached was because they hated the media, especially after being publicly blasted on a UK games show. The other reason was that they did not have enough staff to be able to attend game shows or showcase products, all their time was spent in development.
Rare's lack of quality titles since joining Microsoft has nothing to do with them being shy either. Most of the talent that made the great N64 games had left Rare before Microsoft bought them, so they basically just bought the name while the talent had already left to join Nintendo, Sony and the rest formed Free Radical.