Of course Issac is going to tell us that Sacramental Baptism is the only way Original Sin can be remitted and he'll tell us the below quote is not infallible, yet he can't explain why Pope Pius XII would mention this truth in expaining the absolute necessity of Baptism for an infant of someone who has no use of reason.
Supernatural life
If what We have said up to now concerns the protection and care of natural life, much more so must it concern the supernatural life, which the newly born receives with Baptism. In the present economy there is no other way to communicate that life to the child who has not attained the use of reason. Above all, the state of grace is absolutely necessary at the moment of death without it salvation and supernatural happiness—the beatific vision of God—are impossible. An act of love is sufficient for the adult to obtain sanctifying grace and to supply the lack of baptism; to the still unborn or newly born this way is not open. Therefore, if it is considered that charity to our fellowman obliges us to assist him in the case of necessity, then this obligation is so much the more important and urgent as the good to be obtained or the evil to be avoided is the greater, and in the measure that the needy person is incapable of helping or saving himself with his own powers; and so it is easy to understand the great importance of providing for the baptism of the child deprived of complete reason who finds himself in grave danger or at death's threshold.
Undoubtedly this duty binds the parents in the first place, but in case of necessity, when there is no time to lose or it is not possible to call a priest, the sublime office of conferring baptism is yours.
Here's a few quotes from that are of the same authority as the one you quote, except they say the opposite;
Pope Pius XI, Ad Salutem (#44), April 20, 1930: “Some bishops and priests were at a loss as to what course to pursue in the midst of so many crushing disasters. One of them asked Augustine his opinion: ‘Surely we know [Augustine said] that when such perils reach their crest and no escape is possible, people of both sexes of all ages are wont to flock to the Church.
Some beg for baptism, some for reconciliation, some for the performance of penance, all for consolation and the sacraments to be made available and administered. In such a crisis, if ministers be lacking,
utter ruin is the lot of those who leave this world unregenerated or unshriven.’”
Pope St. Siricius, Letter to Himerius, 385:
“As we maintain that the observance of the holy Paschal time should in no way be relaxed, in the same way we desire that infants who, on account of their age, cannot yet speak,
or those who, in any necessity, are in want of the water of holy baptism, be succored with all possible speed, for fear that, if those who leave this world should be deprived of the life of the Kingdom for having been refused the source of salvation which they desired, this may lead to the ruin of our souls. If those threatened with shipwreck, or the attack of enemies, or the uncertainties of a siege, or those put in a hopeless condition due to some bodily sickness, ask for what in their faith is their only help, let them receive at the very moment of their request the reward of regeneration they beg for. Enough of past mistakes! From now on, let all the priests observe the aforesaid rule if they do not want to be separated from the solid apostolic rock on which Christ has built his universal Church.”
St. Ambrose: "You have read, therefore, that the three witnesses in Baptism are one: water, blood, and the spirit; and if you withdraw any one of these, the Sacrament of Baptism is not valid. For what is water without the cross of Christ? A common element without any sacramental effect. Nor on the other hand is there any mystery of regeneration without water: for ‘unless a man be born again of water and the Spirit, he cannot enter the kingdom of God.’ [John 3:5]
Even a catechumen believes in the cross of the Lord Jesus, by which also he is signed; but, unless he be baptized in the name of the Father and of the Son and of the Holy Spirit, he cannot receive the remission of sins nor be recipient of the gift of spiritual grace."Pius XII also said the belief in evolution is alright and that people may limit the births of their children. He was not a very good pope. The point is that we must believe what we are taught from the popes infallibly.
Pope Paul III, Council of Trent, Sess. 5, on original sin, ex cathedra: "If any one asserts,
that this sin of Adam,--which in its origin is one, and being transfused into all by propogation, not by imitation, is in each one as his own, --
is taken away either by the powers of human nature, or by any other remedy than the merit of the one mediator, our Lord Jesus Christ, who hath reconciled us to God in his own blood, made unto us justice, santification, and redemption;
or if he denies that the said merit of Jesus Christ is applied, both to adults and to infants, by the sacrament of baptism rightly administered in the form of the church; let him be anathema: For there is no other name under heaven given to men, whereby we must be saved. Whence that voice; Behold the lamb of God behold him who taketh away the sins of the world; and that other;
As many as have been baptized, have put on Christ."