The teaching quoted by the OP (Cantate Domino, from the 11th Session of the Council of Florence) is infallible, because it forms part of the Ordinary Infallible Magisterium.
Lumen Gentium, the Dogmatic Constitution on the Church, states:
"Although the individual bishops do not enjoy the prerogative of infallibility, they nevertheless proclaim Christ’s doctrine infallibly whenever, even though dispersed through the world, but still maintaining the bond of communion among themselves and with the successor of Peter, and authentically teaching matters of faith and morals, they are in agreement on one position as definitively to be held.(40) This is even more clearly verified when, gathered together in an ecumenical council, they are teachers and judges of faith and morals for the universal Church, whose definitions must be adhered to with the submission of faith."* (n. 25)
Clearly, the statement contained in the first post is an exercise of this Ordinary Infallible Magisterium. It expounds something which has always been present in the deposit of faith and has been taught and believed universally since apostolic times. It would be a falsification of history to suggest that there was anything short of unanimity on this doctrine on the part of the Church’s Bishops at the Council of Florence. There is no doubt whatsoever concerning the infallibility of this teaching.
Since it forms part of the Church’s Infallible Magisterium, this means that it is irreformable. It cannot be superseded by any subsequent act of the Magisterium.