Okay, I guess I see how you could view it that way. However, the actual wording in Pope Pius IX’s Ineffibilus Deus is that Mary was free of original sin “by a special privilege of God, in view of the merits of Jesus Christ” and in another section the wording is “by virtue of the foreseen merits of Christ”. Since Jesus Christ already existed before his incarnation as human Jesus, I’m not sure if we’re seeing a retroactive effect removing Mary’s sin here, although the term “foreseen merits” does perhaps suggest a reference to the future sacrifice of human Jesus.
Also, to the degree her Immaculate Conception was a “special privilege of God”, then there was no retroactivity needed as God is outside of time.
Having said that, I do believe it’s possible that the Fall of Man could have a retroactive effect.