What is your objection to this statement? The Theotokos was still needed to be rescued by Christ because even though she is pure and immaculate, she is not exempted from the state of the nature she is born into. What you have stated is not against what has been defined in the Immaculate Conception and is in fact consistent with how the faith is expressed and defined.
If you read the statement through you will understand it rejects the immaculate conception on the basis that if it is true then Mary would not have to be saved. The fact that St Thomas at least in his Summa did not believe in the Immaculate conception is not up for discussion, it is a
fact
‘**St. Thomas at first pronounced in favour of the doctrine in his treatise on the “Sentences” (in I. Sent. c. 44, q. I ad 3), yet in his “Summa Theologica” he concluded against it. Much discussion has arisen as to whether St. Thomas did or did not deny that the Blessed Virgin was immaculate at the instant of her animation, and learned books have been written to vindicate him from having actually drawn the negative conclusion. Yet it is hard to say that St. Thomas did not require an instant at least, after the animation of Mary, before her sanctification. His great difficulty appears to have arisen from the doubt as to how she could have been redeemed if she had not sinned. This difficulty he raised in no fewer than ten passages in his writings (see, e.g., Summa III:27:2, ad 2). But while St. Thomas thus held back from the essential point of the doctrine, he himself laid down the principles which, after they had been drawn together and worked out, enabled other minds to furnish the true solution of this difficulty from his own premises. **’ Catholic Encyclopedia
We can clearly see he did not believe in it here '**I answer that, The sanctification of the Blessed Virgin cannot be understood as having taken place before animation, for two reasons. First, because the sanctification of which we are speaking, is nothing but the cleansing from original sin: for sanctification is a “perfect cleansing,” as Dionysius says (Div. Nom. xii). Now sin cannot be taken away except by grace, the subject of which is the rational creature alone. Therefore before the infusion of the rational soul, the Blessed Virgin was not sanctified.
Secondly, because, since the rational creature alone can be the subject of sin; before the infusion of the rational soul, the offspring conceived is not liable to sin. And thus, in whatever manner the Blessed Virgin would have been sanctified before animation, she could never have incurred the stain of original sin: and thus she would not have needed redemption and salvation which is by Christ, of whom it is written (Matthew 1:21): “He shall save His people from their sins.” But this is unfitting, through implying that Christ is not the “Saviour of all men,” as He is called (1 Timothy 4:10). It remains, therefore, that the Blessed Virgin was sanctified after animation.
Reply to Objection 1. The Lord says that He “knew” Jeremias before he was formed in the womb, by knowledge, that is to say, of predestination: but He says that He “sanctified” him, not before formation, but before he “came forth out of the womb,” etc. As to what Ambrose says, viz. that in John the Baptist there was not the spirit of life when there was already the Spirit of grace, by spirit of life we are not to understand the life-giving soul, but the air which we breathe out [respiratus]. Or it may be said that in him as yet there was not the spirit of life, that is the soul, as to its manifest and complete operations.
Reply to Objection 2. If the soul of the Blessed Virgin had never incurred the stain of original sin, this would be derogatory to the dignity of Christ, by reason of His being the universal Saviour of all. Consequently after Christ, who, as the universal Saviour of all, needed not to be saved, the purity of the Blessed Virgin holds the highest place. For Christ did not contract original sin in any way whatever, but was holy in His very Conception, according to Luke 1:35: “The Holy which shall be born of thee, shall be called the Son of God.” But the Blessed Virgin did indeed contract original sin, but was cleansed therefrom before her birth from the womb. This is what is signified (Job 3:9) where it is written of the night of original sin: “Let it expect light,” i.e. Christ, “and not see it”–(because “no defiled thing cometh into her,” as is written in Wisdom 7:25), “nor the rising of the dawning of the day,” that is of the Blessed Virgin, who in her birth was immune from original sin.
Reply to Objection 3. Although the Church of Rome does not celebrate the Conception of the Blessed Virgin, yet it tolerates the custom of certain churches that do keep that feast, wherefore this is not to be entirely reprobated. Nevertheless the celebration of this feast does not give us to understand that she was holy in her conception. But since it is not known when she was sanctified, the feast of her Sanctification, rather than the feast of her Conception, is kept on the day of her conception.
Reply to Objection 4. Sanctification is twofold. One is that of the whole nature: inasmuch as the whole human nature is freed from all corruption of sin and punishment. This will take place at the resurrection. The other is personal sanctification. This is not transmitted to the children begotten of the flesh: because it does not regard the flesh but the mind. Consequently, though the parents of the Blessed Virgin were cleansed from original sin, nevertheless she contracted original sin, since she was conceived by way of fleshly concupiscence and the intercourse of man and woman: for Augustine says (De Nup. et Concup. i): “All flesh born of carnal intercourse is sinful.” **’
