Not sure how the Africans refusing to accept Pope Zosimus papal letter declaring Pelagian orthodox fits in here.
Here is what Augstine really thought
Augustine held the Roman church and its bishop in high regard, but he had a non-papal view of church government. Roman Catholic historian Robert Eno comments:
“Elsewhere I have argued in detail Augustine’s views of authority in the Church and that, in my opinion, the council [not the Pope] was the primary instrument for settling controversies…I believe that Augustine had great respect for the Roman church whose antiquity and apostolic origins made it outshine by far other churches in the West. But as with Cyprian, the African collegial and conciliar tradition was to be preferred most of the time.” (The Rise of the Papacy [Wilmington, Delaware: Michael Glazier, 1990], p. 79)
Protestant historian Philip Schaff wrote:
“Augustine, it is true, unquestionably understood by the church the visible Catholic church, descended from the apostles, especially from Peter, through the succession of bishops; and according to the usage of his time he called the Roman church by eminence the sedes apostolica. **But on the other hand, like Cyprian and Jerome, he lays stress upon the essential unity of the episcopate, and insists that the keys of the kingdom of heaven were committed not to a single man, but to the whole church, which Peter was only set to represent. With this view agrees the independent position of the North African church in the time of Augustine toward Rome, as we have already observed it in the case of the appeal of Apiarius, and as it appears in the Pelagian controversy, of which Augustine was the leader. This father, therefore, can at all events be cited only as a witness to the limited authority of the Roman chair. **And it should also, in justice, be observed, that in his numerous writings he very rarely speaks of that authority at all, and then for the most part incidentally; showing that he attached far less importance to this matter than the Roman divines.” (The Master Christian Library [Albany, Oregon: AGES Software, 1998], History of the Christian Church, Vol. 3, p. 246)
As Augustine said:
“Wherefore, if Peter, on doing this, is corrected by his later colleague Paul, and is yet preserved by the bond of peace and unity till he is promoted to martyrdom, how much more readily and constantly should we prefer, either to the authority of a single bishop, or to the Council of a single province, the rule that has been established by the statutes of the universal Church?..[quoting Cyprian] For no one of us sets himself up as a bishop of bishops, or, by tyrannical terror, forces his colleagues to a necessity of obeying, inasmuch as every bishop, in the free use of his liberty and power, has the right of forming his own judgment, and can no more be judged by another than he can himself judge another. But we must all await the judgment of our Lord Jesus Christ, who alone has the power both of setting us in the government of His Church, and of judging of our acts therein.’ Now let the proud and swelling necks of the heretics raise themselves, if they dare, against the holy humility of this address [of Cyprian].” (On Baptism, Against the Donatists, 2:2-4)
**“Well, let us suppose that those bishops who decided the case at Rome were not good judges; there still remained a plenary Council of the universal Church, in which these judges themselves might be put on their defense; so that, if they were convicted of mistake, their decisions might be reversed.” **(Letter 43:19)
Even Catholic historians agree that Augustine did not see Rome’s infallible authority.
Apparently they had no concept of Romes infallible authority.