I guess Pius didn't read the Confederate vice president's defense of slavery and the Confederacy which made reference to social-Darwinist modern pseudo-scientific justification for the institution of slavery.
It was a mixed bag. The Pope was in favor of an agrarian life-style that he felt was more humane and, like many people around him, felt that African Americans needed a paternalistic treatment to be introduced to American life. Granted, they were being brutalized, but both the Pope and, before him, Jefferson regretted this and felt that while slavery was not in itself good, the African slaves had been brought to this country and were an established fact here and therefore it was the responsibility of the people who had brought them here to maintain them and educate them so that one day they could be fully mature in American lifee.
Interestingly, the Protestants had a special theory to justify slavery. It was called dual creationism, and the theory was that God had created whites first and then created blacks to be their servants.
The Catholic bishop here in St. Augustine was a Frenchman, Agustin Verot, who had been on the side of the Confederacy during the Civil War. St A was not a diocese at that point and he was here as the vicar for the bishop of Savannah, only becoming bishop after the Civil War, in 1870 when St A became a diocese.
When the Confederacy lost, Verot adapted and actually even brought in a religious order to teach black children. These children were virtually adrift because they had never been educated, many had been sold without their families or had been sold with only their mothers and in any case had never received any education or training for living as non-slaves and independent workers in an employee culture.
When Verot went to Vatican I in 1869, before St A became a diocese, he went with two ideas: the Church should reject the “dual creation” theory; and the Church should have a positive attitude towards science.
The latter was well received by the heirarchs in attendance, but the rejection of “dual creation” theory never even came to a vote because it was something totally unknown to the European Catholic cardinals and bishops and, in fact, to Europeans in general. They had no idea what Bishop Verot was talking about and simply moved on to other things.
Actually, all of these matters had been resolved when Spain started exploring places where suddenly Europeans found people who didn’t look or act like them. It was resolved at that time, about 3 centuries earlier, that they were all human beings. But since Protestants didn’t accept Rome’s teachings in the first place and also had no central authority of their own who could tell them how to deal with this, regional and “ecclesial” (that is, one Protestant sect as opposed to another) interpretations sprang up, and this was a significant factor in confusing the way that the young US dealt with slavery.
Maybe he did read the part about rejecting an all powerful federal government?