The Catholic Church most certainly did hold that slavery was moral. One of the Lateran Councils said that those who aided the Muslims were to be reduced to perpetual slavery. Various Church official, up to and including popes, owned slaves.

You traditionalists pretend that the magisterium never accepted the morality of slavery, you do this in one of two ways: You either outright lie about it, or else you play dishonest word games. You make it quite clear that you would embrace dishonesty rather than admit the fact that the magisterium can and has changed its teachings.