Curious, do you have a source for this? I must admit I'm puzzled as to how Bl. Pius IX knew this.
Yes, where did the Pope go to get this information?
But he could have known this in virtue of the authority to bind and loose.
You shouldn't have been. Ever heard of binding and loosening?
Pope Bl. Pius IX in Apostolicae Nostrae Caritatis:
"Indeed We have decided to offer the treasures of divine gifts, whose dispensation the Most High entrusted to Our care, so that your faithful may pray with more fervent love and more abundant fruit. For this reason, by the mercy of almighty God and trusting in the authority of His blessed apostles Peter and Paul, We confer, by the power of binding and loosing which the Lord gave to Us, a plenary indulgence of all sins as in a jubilee. This indulgence will be granted to all the faithful of your dioceses who accomplish the following acts within a three month span to be determined by each one of you and computed from the day which you decide upon. They must have humbly confessed their sins with sincere regret for them, and after being purified with sacramental absolution, they must have reverently received the sacrament of the Eucharist. They must also visit three churches designated by you or one of the churches three times and must say prayers there for the exaltation and prosperity of Holy Mother Church and the Apostolic See, for the elimination of heresies, for the peace and harmony of the Christian princes, and for the tranquillity and unity of the whole Christian people. Moreover they should fast once within the same period and give some alms to the poor. This indulgence can also be applied to the souls in purgatory."
There you go.
He doesn't have to "know" it. As the Vicar of Christ on earth, the visible head of Holy Mother Church, with the authority to bind and loose, he alone has the authority to grant plenary indulgences and set the conditions there of. Which of course is just a drawing on the spiritual treasure of the Church made up of the superabundant merits of Our Lord, Our Holy Mother and the saints. The merits of the Passion and death of Our Lord are infinite, and these he left to the Church.
But of course, as a well catechized Catholic, I'm sure you knew this.