Indeed they aren't. St. Augustine's teaching on the Original Sin is what is called Augustinianism; the Catholic doctrine on Original Sin was in consensus with the Greek Fathers prior to that, and returned to essentially the same teaching with St. Thomas Aquinas.
The term "Augustinian" is frequently used in the same sense I use it, not to discard the immense work of St. Augustine's overall, but to point out elements of his teaching that fell outside of the approved doctrine, by the Catholic Encyclopedia article I am referring to, Limbo.
The Catholic Church essentially accepts St. Augustine's teachings on Original Sin as he formulated it. And these teachings are in consensus with the Greek Fathers and St. Thomas Aquinas.
See Denzinger 101 and 102 (Council of Carthage, AD 418), 109 (Epistle of Pope Zozimus to the Eastern Churches, AD 418), 130 (Catalog of Authoritative Statements of the Past Bishops of the Holy See Concerning the Grace of God), 174 and 175 (Council of Orange).
Similarly, there is the pronouncement of Pope St. Gelasius I:
"For the purpose of instruction, the holy, that is, the Roman Church, does ... regularly accept ... the works of blessed Caecilius Cyprian and in the same way the works of Gregory Nazianzen, Basil, Athanasius, John (Chrysostom), Theophilus, Cyril of Alexandria, Hilary, Ambrose, Augustine, Jerome, and Prosper" (Epistle 42, "De recipiendis et non recipiendis libris", AD 495)