Sedevacantists and traditionalists should check the websites they follow and ask where is B an exception to A.They will find many examples of this irrationality in the interpretation of Vatican Council II.
For example Lumen Gentium 8 'subsist it', can it be exception to the orthodox passages in Ad Gentes 7 and Unitatitis Redintigratio 3 ? No! Yet this is the misake made by bishops and priests.
They should also ask where in the writings of the Church Fathers, Church Councils and saints before 1949 is it mentioned that the baptism of desire and being saved in invincible ignorance, refer to explicit cases and that they are exceptions to the dogma extra ecclesiam nulla salus? There are none. This is also a common error on Catholic websites.