First, to believe in BOD / BOB for catechumen / martyrs is not a heresy, given that there has clearly been Church teaching on them. But BOD/BOB have never been dogmas, only theories that belong to the realm of speculation. Because they are fallible, they can be change, and so they should, since BOD is the loophole the modernist liberals have used to undermine the exclusivity of the Catholic Church as only means of salvation and deny EENS.
Again, the discussion is not about BOD/BOB really but salvation of non-Catholics.
Second, what is the merit of being a Catholic if also a Muslim can be saved? Imagine what had become of Our Catholic religion if the great conquistadores and evangelists had held the heresy of invincible ignorance. There had not been any need to convert the pagans or the natives. It is very simple, indeed. There had not ever been Christendom nor civilization. All of the worthy missionaries, valiant crusaders, courageous martyrs, did what they did because they wholeheartly believed in the exclusivity of the Catholic Faith for salvation. None of them undermined or denied EENS.
Third, reflect on this picture about the invincible ignorant. Perhaps it will demonstrate what is it that we are talking about.