No, I believe the IC is a Pelagian heresy. Among the ancient church, the only ones teaching a form of sinlessness were the Pelagians. None of the church fathers taught that Mary was immaculately conceived. So the claim of Ineffabilis Deus regarding the nature of this teaching in church history is basically a false one.
You wrote:
“No, I believe the IC is a Pelagian heresy.”
Which makes absolutely no sense. In a miracle at conception no human works are even possible.
“Among the ancient church, the only ones teaching a form of sinlessness were the Pelagians.”
That again makes no sense. You’re claiming that a doctrine held only by orthodox Christians is related to a doctrine held only by heretics. That in itself seems like an impossibility. Also, this was a singular grace of God in Mary’s case and that is NOT what the Pelagians believed in. Your views are incoherent.
“None of the church fathers taught that Mary was immaculately conceived.”
I think you should look into that claim. How about Proclus of Constantinople:
As He formed her without any stain of her own, so He proceeded from her contracting no stain (Homily 1[ante A.D. 446]).
Jacob of Sarug:
[T]he very fact that God has elected her proves that none was ever holier than Mary, if any stain had disfigured her soul, if any other virgin had been purer and holier, God would have selected her and rejected Mary[ante A.D. 521].
Romanos the Melodist:
Then the tribes of Israel heard that Anna had conceived the immaculate one. So everyone took part in the rejoicing. Joachim gave a banquet, and great was the merriment in the garden. He invited the priests and Levites to prayer; then he called Mary into the center of the crowd, that she might be magnified (On the Birth of Mary 1 [d. ca A.D. 560]).
Some Fathers clearly taught she was sinless in their writings. So, when do you think they believed she became immaculate?
Ambrose of Milan
Come, then, and search out your sheep, not through your servants or hired men, but do it yourself. Lift me up bodily and in the flesh, which is fallen in Adam. Lift me up not from Sarah but from Mary, a Virgin not only undefiled but a Virgin whom grace had made inviolate, free of every stain of sin (Commentary on Psalm 118:22-30 [A.D. 387]).
“So the claim of Ineffabilis Deus regarding the nature of this teaching in church history is basically a false one.”
No. Only your understanding is false. Anti-Catholics rarely understand orthodox Christianity since it is foreign to them.