Dimond brothers are setting up something that isnt De Fide to be so, condemning others who don't.
To quote from their website: "There is only one baptism, which is celebrated in water (de fide)." I won't bother linking to that poisonous well.
What is De Fide? "A term meaning "of Faith," used to identify those doctrines of the Church which are infallibly true."
According to original 1952 'Fundamentals of Catholic Dogma' by Ludwig Ott (Priest and Prof. in Dogmatics) which has a Nihil Obstat and Imprimatur given in 1954, Baptism of Desire is a doctrine that is sententia fidei proxima which means "A Teaching proximate to Faith (sententia fidei proxima) is a doctrine, which is regarded by theologians generally as a truth of Revelation. but which have not yet been finally promulgated as such by the Church."
Something that contradicts this would be censured as "a Proposition Proximate to Heresy (propositio heresi proxima) which signifies that the proposition is opposed to a truth which is proximate to the Faith (Sent. fidei proxima)", that is, an opinion approaching heresy.
Pope Innocent III "Apostolicam Sedem" to the Bishop of Cremona on the subject of Baptism of Desire:
To your inquiry we respond thus: We assert without hesitation (on the authority of the holy Fathers Augustine and Ambrose) that the priest whom you indicated (in your letter) had died without the water of baptism, because he persevered in the faith of holy mother the Church and in the confession of the name of Christ, was freed from original sin and attained the joy of the heavenly fatherland. Read, brother, in the eighth book of Augustine's "City of God" * where among other things it is written, "Baptism is ministered invisibly to one whom not contempt of religion but death excludes." Read again the book also of the blessed Ambrose concerning the death of Valentinian where he says the same thing. Therefore, to questions concerning the dead, you should hold the opinions of the learned Fathers' and in your church you should join in prayers and you should have sacrifices offered to God for the priest mentioned."
- Denzinger's "The Sources of Catholic Dogma" Nihil Obstat and Imprimatur 1955