I've yet to see any magisterial statement condemning belief in Baptism of Desire as heretical. It's not like theologians haven't taught it for centuries, before Vatican II, without censure.
If it were an erroneous interpretation of the teaching on Baptism, there would have been a clear condemnation of it, explicitly. The Church had ample opportunity to muster up a condemnation before Vatican II happened.
Pure speculation on your part.
Did the Church explicitly condemn the teaching that Our Lady was conceived with original sin or did the Church, without regard to the teaching or persons teaching that error, simply declare the infallible truth?
Likewise the Church did not need to explicitly condemn a BOD when She infallibly declared the sacraments are a necessity for salvation - that decree alone is enough to condemn a BOD, combine that with her numerous other infallible decrees on the necessity of the sacrament, and only the fool continues to insist the Church did not mean what she infallibly and clearly declared.
Also, from the time of Augustine (4th century) to Abelard (12th century) it was the common and almost unanimous teaching of theologians that unbaptized infants suffer the fires of Hell after death, a position that was later condemned by Pope Pius VI. This proves that the “common” error of one period (or even for hundreds of years) is not the universal and constant teaching of the Church from the beginning.
The Catholic Encyclopedia, Vol. 9, “Limbo,” p. 257: “After enjoying several
centuries of undisputed supremacy, St. Augustine’s teaching on original sin was first successfully challenged by St. Anselm, who maintained that it was not concupiscence, but the privation of original justice, that constituted the essence of inherited sin. On the special question, however, of the punishment of original sin after death, St. Anselm was at one with St. Augustine in holding that unbaptized infants share in the positive sufferings of the damned; and Abelard was the first to rebel against the severity of the Augustinian tradition on this point.”