"in Whose Birth His Mother's virginity remained inviolate"
That says nothing about perpetual virginity. All Augustine is saying is that the Holy Spirit came over Mary and at the time of Jesus' birth she was a virgin.
The Fifth Ecumenical Council (533 AD) does say that Mary was ever-virgin, i.e. perpetually virgin.
-A8
More precisely he is saying that her virginity "remained inviolate" following the Nativity, but this is all that is being argued at the moment: that per St. Augustine Jesus was not only conceived miraculously but also born miraculously.
Would it be your contention that St. Augustine did not believe in the Blessed Virgin's perpetual virginity even though he believed in her virginity following giving birth?