You wrote:
“True, but he is clearly not coming out an affirming the post-medieval RC doctrine of the IC.”
It’s not post medievel.
“He asks, “how do we know?” Well, the point is, according to Augustine, we don’t. Certainly not to the point of making it dogma of the church.”
That’s not what he is saying.
See How Many Popes Does it Take to Deny the Immaculate Conception?
That’s not what he is saying.
Clearly it is. Augustine would not teach a Pelagian doctrine like the IC.
Moreover, when expounding the Gospel according to Luke, he [i.e. Ambrose] says: "It was no cohabitation with a husband which opened the secrets of the Virgin's womb; rather was it the Holy Ghost which infused immaculate seed into her unviolated womb. For the Lord Jesus alone of those who are born of woman is holy, inasmuch as He experienced not the contact of earthly corruption, by reason of the novelty of His immaculate birth; nay, He repelled it by His heavenly majesty." (Augustine's Anti-Pelagian Works quoted here )