OK, please try to follow the reason, would you, children?
Papal legitimacy is dogmatic fact.
Consequently, papal legitimacy must be known with the certainty of faith.
But +Williamson and +Lefebvre said it's possible that they have not been legitimate.
Consequently, their legitimacy lacks the certainty of faith.
Consequently, they are not sedeplenists in the Catholic sense, but sede-doubtists.