The tradition of the Catholic Church says that, barring a revelation from God, we do not know for sure where they or anybody else for that matter goes when they die.
Incorrect extrapolation to a false principle. It is true that we cannot know for sure whether any particular individual has been lost because it's possible that the person received the grace to reject his heresy before death. In that case, the person would not have died a heretic. So it's a moot point.