Fr. Laisney’s argument is two fold.
1. Since Cantate Domino says “regarding children since the only remedy...” which implies that another remedy is available to adults i.e Baptism of Desire. (See full quote in OP)
2. Cantate Domino is quoting St. Fulgentius and St. Thomas Aquinas in the passages cited against belief in Baptism of Blood and Baptism of Desire respectively yet St. Fulgentius goes on to mention Baptism of Blood in what he is quoted in and St. Aquinas the same for Baptism of Desire.
I’m curious to know if Fr. Feeney or someone who holds to his position has addressed this argument before hence my OP. I couldn’t find anything on it.
You (and Laiseney) left out the part where St. Fulgentius rejects Baptism of Desire. By Laisney's own (dishonest) argument, this "proves" that Florence rejected Baptism of Desire.

Sorry, but that's one of the dumbest things I've ever heard, that a citation from a Council of some passage from a saint means that the Council is teaching everything the cited author teaches about the subject.