Ridiculous. Fr. Garrigou-Lagrange notes
"Theologians in general are inclined to fill out what Scripture and tradition tell us by distinguishing the means of salvation given to Catholics from those that are given men of good will beyond the borders of the Church. …If we are treating of all Christians, of all who have been baptized, Catholic, schismatic, Protestant, it is more probable, theologians generally say, that the great number is saved. First, the number of infants who die in the state of grace before reaching the age of reason is very great. Secondly, many Protestants, being today in good faith, can be reconciled to God by an act of contrition, particularly in danger of death. Thirdly, schismatics can receive a valid absolution"
By an act of contrition it says. To be contrite, one must know that one has done wrong in the first place and be sorry for it. Therefore, for one to be contrite about being a Protestant, they must realise that Protestantism is heresy and wish to abandon that heresy.
Only way around it I could see is a perfect act of contrition, such as martyrdom in the name of the Lord, whereby all sins are forgiven absolutely. Protestants being already baptised, could perhaps then be saved. Although not pagans or Jews.
Also I don't think there's a consensus at all that the great number of Catholics(let alone heretics!) are saved. More commonly I've heard that even most Catholics are damned, and surely even if heretics were saved it'd be much smaller proportion again who'd be saved.