See if you can understand this:
.....For the rest, We cannot hide from you that We have been made rather anxious: for We feared that the example of this Congress, assembled independently of the ecclesiastical authority, might little by little do damage to the right of spiritual government and legitimate teaching which, in virtue of the divine institution, belongs properly to the Roman Pontiff and to the bishops who in union and agreement with the Successor of St. Peter; and that, as a consequence of this harm done to the government of the Church, the principle of unity and obedience in matters of faith might eventually be weakened in many souls. We feared also lest, in the same Congress, opinions and systems might be aired and supported which, by reason above all of the publicity given to them, would imperil the purity of doctrine and the duty of obedience.
1) Now if anything that is taught from the ordinary universal magisterium is guaranteed to be free from the possibility of error - then there would have been zero reason for Pope Pius IX to even write the letter.
2) The fact that he even wrote the letter serves as proof that, per the letter itself, the UOM can err.
3) Why would the pope have
"been made rather anxious" at all if the ordinary universal magisterium is incapable of teaching error?
4) Why would he fear that the ordinary universal magisterium,
"might little by little do damage to the right of spiritual government and legitimate teaching" if it is a teaching of the Church that the ordinary universal magisterium cannot err in it's teaching?
5) The reason the letter was written at all, and the reason the pope was made rather anxious and the reason he was afraid (
"We feared also") was because the teaching authority that the ordinary universal magisterium have, can be abused.
"We feared also lest, in the same Congress, opinions and systems might be aired and supported which, by reason above all of the publicity given to them, would imperil the purity of doctrine and the duty of obedience." 6) All you need to accept is that he never would have had any need to write the letter if the guarantee of infallibility, automatically extended to the ordinary universal magisterium!
7) Try and remember that from now on!