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.