Wow! Thank you for sharing. Fr. Hardon is considered by many to be the definitive catechist so his words carry weight beyond a regular priest.
The moral - but not metaphysical - impeccability of the Blessed Virgin Mary is a traditional aspect of Roman Catholic Mariological teaching. Here are a few additional quotations on the topic from other reputable theologians of the Roman Catholic Church:
"No one can attain the Love of God without humility; and that is a gift of God above all temporal gifts. Humility brings true peace to the heart of man; for no one is quarrelsome or sinful, but he who lacks peace. If a man were truly humble he would never sin again. Mary could not sin because she was truly humble; and wherever God finds humility, there He does great things. Augustine says: 'The lowliest on earth is also the holiest.'"
Johannes Tauler, OPThe Inner Way
"According to the teaching of the Church and the nature of the matter, the fact of Mary's sinlessness is based on a special and supernatural grace, or rather on an exceptional privilege of grace. For the reason that this grace made complete sinlessness possible, it was called 'the gift of impeccancy' as in the first couple, and its main feature is freedom from concupiscence. In so far as it ensured effective freedom from sin, it was called, in relation to mortal sins, the 'gift of confirmation in grace' or of 'perfect perseverance'; in relation to venial sins, the 'gift of perfect confirmation in good.' In this respect its main characteristic is the fullness of the graces of supernatural sanctification, actual as well as habitual, whereby God, who was with and in Mary in a special manner, so guarded her that she never committed a single sin.
The precise meaning of this grace and its manner of action are linked with the question, whether the confirmation in grace abolished not only the reality but also the possibility of sin, or the reality by the very impossibility, in the same way as the inclination to irregular motions of concupiscence, and thereby to formal sin, was eradicated by the extinction of the fomes.
As clear as it is that Mary is distinguished from God and Christ by the fact that, considered in her being, she was capable of sin, it is just as clear that, because of the divine protection and assistance which Mary enjoyed, it can and must be said in a certain sense that she could not have sinned, that in her, sin was impossible, hence her sinlessness includes the incapability of sinning. This, in a sense at least, is analogous to what, relative to the teaching authority of the Church, is said of the pope, that he cannot fail in certain acts and that his freedom from error includes infallibility.
In fact, Mary's incapability of sinning, as the pillar and vesture of her sanctity, is the original type and guarantee of the infallibility of the Church, like a pillar and vesture of truth. Just as the Church's truth rests on the words, 'I am with you all days,' so Mary's sanctity rests on 'The Lord is with thee.' Both are prophesied in a typical sense by the words: 'The stream of the river maketh the city of God joyful: the most High hath sanctified His own tabernacle. God is in the midst thereof, it shall not be moved.'"
Fr. Matthias J. ScheebenMariology
(St. Louis: B. Herder Book Company, 1948)
"Impeccability may be either metaphysical or moral. Metaphysical impeccability belongs exclusively to God, whereas moral impeccability may also be enjoyed by creatures. It is enjoyed, e.g., by the angels and saints in Heaven. God is impeccable because He is absolutely and infinitely holy; Christ, in consequence of the Hypostatic Union; the angels and saints, by virtue of the beatific vision of the Godhead which they enjoy. How are we to conceive of the impeccability of the Blessed Virgin Mary? It is quite obvious that her impeccability must differ specifically from that proper to God and the God-man Jesus Christ. Here is not a divine attribute, nor is it conditioned by or based upon a personal union of divinity with humanity. It cannot be a result of the beatific vision, because Mary during her sojourn on earth was a wayfarer like ourselves and did not enjoy beatitude. Comparing her impeccability to that of the angels and saints and to that of our first parents in Paradise, we may define it as an intermediate state between the two. It would be asserting too much to say that the Blessed Virgin was capable of committing sin like our first parents; and too little to assert that during her life-time she was incapable of sinning as the angels and saints of Heaven are now, in consequence of the beatific vision. In what, then, did her impeccability consist? We are probably not far from the truth when we assume that God gave her the gift of perfect perseverance as against mortal sin, and that of confirmation in grace as against venial sin. Together with her freedom from concupiscence these two graces may be regarded as the proximate cause of Mary's impeccability. For its ultimate cause we must go back to the higher and more comprehensive prerogative of her divine motherhood. God owed it to His own dignity and holiness, so to speak, to bestow the grace of perfect perseverance and confirmation in grace upon her from whom His Divine Son was to assume human nature. This idea is aptly illustrated by 'the woman clothed with the sun' whom St. John visioned in the twelfth chapter of the Apocalypse. The analogy between Mary's impeccability and that of her Divine Son would seem to render this theory all the more acceptable, though we must, of course, never forget that the impeccability of Christ is based upon the Hypostatic Union of Godhead and manhood, whereas that of His Mother rests merely upon the grace of divine motherhood."
Fr. Joseph PohleDogmatic Theology VI
Mariology: A Dogmatic Treatise on the Blessed Virgin Mary, Mother of God
(St. Louis: B. Herder Book Company, 1916)
"There are two types of impeccability: metaphysical and moral. The former is predicated exclusively of God who is holiness itself, and also of Christ, due to the Hypostatic Union. The latter belongs to the angels and saints on account of the beatific vision, and likewise to Our Blessed Lady, although for reasons other than those mentioned in the case of God, Christ, the angels and saints.
According to Alexander of Hales (d. 1245), Mary was impeccable because of her fullness of grace. According to St. Bonaventure (d. 1274), Mary received a special divine assistance which strengthened the potencies of her soul, and this made it impossible for her to sin. St. Thomas (d. 1274) thought that Mary was impeccable owing to the constant act of divine Providence removing all occasions of sin from her path. Finally, in the opinion of Suarez (d. 1617) and most theologians, the remote cause of Mary's impeccability was the divine Motherhood; the proximate cause was threefold: the lack of concupiscence, the fullness of grace, and an act of divine Providence which not only removed all occasions of sin from her but also confirmed her in grace."
Fr. Juniper B. Carol, OFMFundamentals of Mariology
(New York: Benziger Brothers, Inc., 1956)