>>56630894There is a non-zero chance that the other two starters were "finalized", but when Sakurai didn't pick them the devs went "maybe they're shit, let's go back to the drawing board".
It depends heavily on when Sakurai was shown the mons. Dev cycle on Smash characters is really foggy, and I don't believe Sakurai's commented on timelines in his dev series. They could've had Greninja done in a month, or been working on it a year in advance of release.