I think you misunderstand what is meant by ‘full knowledge’.
A person who is not specifically taught by his or her Catholic parents, teachers, friends or family the whole 6th commandment can still have ‘full knowledge’ that fornication is something ‘wrong’, and thus commit mortal sin.
God doesn’t have two sets of rules for the Christian and the non-Christian, so that the non-Christians who supposedly ‘don’t know the rules’ get by scot-free.
However, might I suggest that rather than ask about a group of people of whom you are not a part yourself, that you focus on your own relationship with God? When you have a real knowledge of your faith under your belt, you’ll be much better able, if God leads you that way, to go considering the putative ‘fate’ of others.