That is the exemption, since those non christians who no fault of their own do not know the Gospel of Christ or His Church, yet sincerely seek God and moved by grace strive by their deeds to do His will as it is known to them through the dictates of conscience. Nor does Divine Providence deny the helps necessary for salvation to those who, without blame on their part, have not yet arrived at an explicit knowledge of God and with His grace strive to live a good life. Whatever good or truth is found amongst them is looked upon by the Church as a preparation for the Gospel., this is called invincible ignorance, meaning it is not their fault that the gospel has not been preached to them. Example if they in their life are seeking to know a God they have not known not contented with the life they have and then suddenly they die. Well of course God will not deny them the salvation they deserve. In fact Saint Paul himself said that:
"For when the Gentiles, who have not the law, do by nature those things that are of the law; these having not the law are a law to themselves: Who shew the work of the law written in their hearts, their conscience bearing witness to them, and their thoughts between themselves accusing, or also defending one another, In the day when God shall judge the secrets of men by Jesus Christ, according to my gospel."
(Romans 2:14-15, Douay Rheims)
That is the exemption, but if the non christian had already heard the gospel truth the still refused to believe in it and refused to be baptized or enter the Catholic Church then they will not be saved, according to Lumen Gentium it says:
"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."
(Lumen Gentium, 14)