To which I reply:a) In this quotation, Augustine is refusing (at the time) to address the question of whether Mary had sin. He does not assert that she was sinless.
b) Augustine is saying that there is one (Jesus Christ) who certainly had no sin.
c) Augustine is addressing the issue of actual sin, not original sin.
Moreover, just a short time before writing "On Nature and Grace," Augustine wrote "On Merits and Forgiveness of Sins," in which he spoke more clearly:Augustine (354-430):
This being the case, ever since the time when by one man sin thus entered into this world and death by sin, and so it passed through to all men, up to the end of this carnal generation and perishing world, the children of which beget and are begotten, there never has existed, nor ever will exist, a human being of whom, placed in this life of ours, it could be said that he had no sin at all, with the exception of the one Mediator, who reconciles us to our Maker through the forgiveness of sins.NPNF1: Vol. V, On Merits and Forgiveness of Sins, and on the Baptism of Infants, Book II, Chapter 47.
Mary was not only free from actual sin, but she was also, by a special privilege, cleansed from original sin. She had, indeed, to be conceived with original sin, inasmuch as her conception resulted from the commingling of both sexes. For the privilege of conceiving without impairment of virginity was reserved exclusively to her who as a virgin conceived the Son of God. But the commingling of the sexes which, after the sin of our first parent, cannot take place without lust, transmits original sin to the offspring. Likewise, if Mary had been conceived without original sin, she would not have had to be redeemed by Christ, and so Christ would not be the universal redeemer of men, which detracts from His dignity. Accordingly we must hold that she was conceived with original sin, but was cleansed from it in some special way. (Thomas Aquinas)Quoted here.