Yoruichi was shown to be a captain of the 2nd Division during the Turn Back the Pendulum chapters (set 110-101 years before Aizen's defection from Soul Society & the battles in the fake Karakura Town). While the "quiz" included in the Bleach Official Bootleg does indeed claim that Yoruichi was not a member of the Gotei 13, Bootleg came out BEFORE the Turn Back the Pendulum chapters. As such, the assistant writers who helped Kubo compile the Bootleg and most likely wrote the quiz, made that mistake. The alternative explanation is that Kubo decided to make Yoruichi a captain after he gave that quiz question the go ahead for publication in Bootlegs.