This is based on certain accounts from people who worked on and listened to these outtakes.how do you know that the outtakes are better? have you heard the ones which aren't released yet?
Like, Bruce Swedien who said that many better, complete songs did not make the 'Invincible' album simply because they did not fit the album's theme and cohesion.
Or, Rodney Jerkins who also stated that they left out better, complete songs than the ones that they eventually included on that album.
It has also been said that Michael Jackson (due to his dissatisfaction and anger with Sony Music at the time) decided not to include some of these better, complete songs on that album, but to keep them for himself so as to release them later on a different record label.
Most of these unreleased outtakes (from that era) apparently do exist.