They deny the son of God, His deity, His crucifixion; it's purpose and deny the Holy Trinity. These are all heresies of a false Gospel.
"I have not written to you because you do not know the truth, but because you do know it, and because no lie is of the truth. Who is the liar but the one who denies that Jesus is the Christ? This is the antichrist, the one who denies the Father and the Son. Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also."
"Jesus said to him, “I am the way, the truth, and the life. No one comes to the Father except through Me."
843 The Catholic Church recognizes in other religions that search, among shadows and images, for the God who is unknown yet near since he gives life and breath and all things and wants all men to be saved. Thus, the Church considers all goodness and truth found in these religions as "a preparation for the Gospel and given by him who enlightens all men that they may at length have life."
845 To reunite all his children, scattered and led astray by sin, the Father willed to call the whole of humanity together into his Son's Church. The Church is the place where humanity must rediscover its unity and salvation. The Church is "the world reconciled." She is that bark which "in the full sail of the Lord's cross, by the breath of the Holy Spirit, navigates safely in this world."
Islam cannot be preparation for the Gospel, since it came
after Christianity. The Jews also held the faith of Abraham, yet their rejection of the Messiah brought salvation to the Gentiles. The establishment of the Church, and the commandment in Scripture, Tradition and the Church to do the work of evangelism was for a reason.
This Sacred Council wishes to turn its attention firstly to the Catholic faithful. Basing itself upon Sacred Scripture and Tradition, it teaches that the Church, now sojourning on earth as an exile, is necessary for salvation. Christ, present to us in His Body, which is the Church, is the one Mediator and the unique way of salvation. In explicit terms He Himself affirmed the necessity of faith and baptism and thereby affirmed also the necessity of the Church, for through baptism as through a door men enter the Church. Whosoever, therefore, knowing that the Catholic Church was made necessary by Christ, would refuse to enter or to remain in it, could not be saved..
So I can only conclude that it means: God can, and does work mercy and salvation in those who through no fault of their own, do not know the way of truth and salvation. It is worded ambiguously, seeming to affirm salvation through Islam. Which of course is
untrue, other than the suggested exception - which should apply to the spectrum of religions.
Those under Islam, as all other religions still need to repent of their heresies and turn to the true faith.
"There is one body and one Spirit, just as also you were called in one hope of your calling; one Lord, one faith, one baptism, one God and Father of all who is over all and through all and in all."
