I can't speak for Xavier, again, but this ultimately comes down to limitations of human language.
It's not a limitation. It's important to be precise. We have dogmatic definitions which state that heretics cannot be saved (many of them in fact). So to assert that heretics can be saved is contrary to Church dogma. Protestants cannot be saved. Are there some living among Protestants who are actually Catholic? Perhaps, though this is likely very rare. They would have to be in either a mental state where they are so befuddled that they don't even know they're Protestant or what Protestant vs. Catholic is. In point of fact, the vast majority of Protestants know the Church and actively (even aggressively) repudiate it. If there are such Catholics among the Protestants, then I am certain that God will ultimately bring them visibly into the Church. We hear thousands upon thousands of conversion stories. God does not generally leave Catholics among the Protestants but brings them into the Church.
Unlike the position that Xavier's pushing, where someone has to actively repudiate the faith, commit an active mortal sin against faith, supernatural faith can be absent by mere privation as well. Going back to the animist baptized in the jungle by a missionary who reaches the age of reason knowing nothing about the faith. He's committed no mortal sin against the faith, but at the age of reason, the infused supernatural faith of Baptism atrophies, as it were, and fades away. According to Xavier's principles, that active sin against the faith is required, this person could reach the age of 30 while continuing to have supernatural faith ... without so much as believing in God. That is not possible. He has not supernatural faith at that point ... even though he's not actively committed any mortal sin against faith.
THAT is the distinction here, that the presence or absence of supernatural faith doesn't necessarily required "bad faith" or "culpability". To suggest that is to flirt with Pelagianism.
Many absurdities flow from Xavier's stated principles. You can have a Protestant who believes that the Catholic Church is the whore of Babylon, who regularly derides Our Blessed Mother, etc., who is perfectly well convinced that he's right (Salvierri's definition of "good faith"). That such person could possibly have supernatural faith is utterly preposterous. But it's a logical conclusion of the narrative being built by Xavier.