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?
"More precisely he is saying that her virginity "remained inviolate" following the Nativity,"
That's not what the quote said. It only addressed virginity up to the time of birth, "in Whose Birth His Mother's virginity remained inviolate". If you have something from Augustine that says differently, let's see it.