The crux of the argument: while baptism of blood and desire exists, both require the use of reason. According to the priest's erroneous line of reasoning, since infants have not the use of reason, aborted infants cannot receive either baptism of blood or baptism of desire.
Furthermore, according to this priest's version of the Feeneyite heresy, if we were to assume that such babies were baptized by blood via their abortion, that would make abortion into a sacrament. The unfortunate blogger, a good man dazed by this dizzying line of clerical absurdity, labels the priest's reductio ad absurdem argument "irrefutable".
Many unfortunate Catholics have fallen under the spell of this particular priest. Even though the blogger takes care to explicitly deny that this nonsense is dogma, his use of the word "irrefutable" implies that the blogger thinks the priest's false line of reasoning is of doctrinal quality.
Unfortunately for both the FSSP priest who teaches this error and the poor Catholics who follow his frequently erroneous reasoning, the liturgy of the Church specifically and explicitly denies the priest's entire line of thought.
Consider the feast of Childermas, the Massacre of the Holy Innocents. Note the word "Holy" in the title of the Feast. Childermas celebrates the entry into heaven of those infants slaughtered by Herod as he fruitlessly sought to murder the King of Kings.
Those infants had not the use of reason. Their only virtue was to be slaughtered for a saviour whom even their own parents did not yet know, yet every irrational infant slaughtered by Herod and baptized only by his own life blood is celebrated by the Church as a saint in the heavenly choir.
Obviously, martyrdom is not a sacrament, yet according to the eternal liturgy of the Church, martyrdom sends people to heaven, even people who have not the use of reason. In fact, martyrdom is so effective at sending infants to heaven, that even the infant's parents need not know anything of Christ in order for the infant, slaughtered as a mute witness, to attain the heavenly kingdom.
Now, if we followed the "very good priest's" logic, we would be forced to conclude that infanticide is a sacrament. But I bet it isn't.
Like infanticide, like abortion, martyrdom is not to be sought, for it may send the one who murders the martyr to hell. Yet martyrdom may also bring about conversion in the murderer, and for this reason, the saint who is faced with martyrdom may embrace it, even embrace it joyfully, as long as he offers his death for the conversion of the one who murders him.
We don't know what happens to innocents who die unbaptized. But the liturgy of Childermas gives us a solid hope, as does the infallible teaching of the Ordinary Infallible Magisterium. Any priest who says otherwise contradicts the infallible Church. Such a priest may be many things, but "good" is not one of them.
When Jesus faced the Sadducees, he faced a group of men who accepted only the Torah as holy Scripture. Much as many traditionalists adhere only to the EF Mass and reject the OF Mass as somehow false or contaminated, the Sadducees refused to accept the prophets or the writings as inspired. Only the eternal Scripture of the Torah was good enough for them.
Jesus used quotes from their own Torah to refute them, and admonished them by saying, "You are quite wrong. You know neither Scripture nor the power of God."
Similarly, the Church, via Her liturgy, rebukes this "good priest" and uses the very EF liturgy he embraces to demonstrate his error. He is quite wrong. He apparently knows neither liturgy nor the power of God.
In fact, the CCC specifically denies that Limbo exists.
CCC 1283 With respect to children who have died without Baptism, the liturgy of the Church invites us to trust in God's mercy and to pray for their salvation.
It is a condemned heresy to say that you can pray for those who are in hell. Limbo, as is commonly envisioned, is the first circle of hell. If Limbo existed, then the CCC would be teaching heresy. The CCC is promulgated by an Apostolic Constitution, the highest expression of the Ordinary Infallible Magisterium. Thus, Limbo does not exist.