The Sedevacantist bishop, Donald Sanborn, concedes that in order for the recent Popes/bishops to lose their office, they would have to be warned and then declared heretics by the Church. Until that happen, he concedes that they legally retain their office.
“…despite his public heresy, it was still necessary that Nestorius undergo warnings by the Pope, and having repudiated the warnings, be officially excommunicated and deposed by the same. The case is strikingly close to our own. … we do not have the authority to declare the sees legally vacant which these heretical ‘popes’ or ‘bishops’ possess de facto. Only the authority of the Church can do that. … until their designation to possess the authority is legally declared null and void by competent authority, the heretical ‘pope’ or ‘bishop’ is in a state of legal possession of the see … He can only lose that state of legal possession by legal deposition.”
We should note that Bishops Sanborn is a different kind of Sedevacantist. He holds to the novel theory that a cleric can legally hold an office, yet not possess the jurisdiction inherent in the office. Based on this, he claims that the recent Popes (and bishops) have all been “legal occupants of their office,” but lack the authority of the office they legally hold. This error of Bishop Sanborn is dealt with in "True or False Pope?"