Not true. The 4 pillars are the minimum to make Nintendo fans happy.
An announcement of a Square Enix RPG or a big name 3rd party game would have done the trick too.
I remember the Capcom announcement for the GC. That did the trick. REmake, RE0, and RE4 all exclusive (or at least timed) for the Cube.
You need to have some kind of marketshare to get the third party support.
If Capcom knew the GameCube was going to sell that poorly they likely never would've made that deal in the first place (as evidence by them trying to weasel out of it later).