ABL:
2. “The question is therefore definitive: is Paul VI, has Paul VI ever been, the successor of Peter? If the reply is negative: Paul VI has never been, or no longer is, pope, our attitude will be that of sede vacante periods, which would simplify the problem. Some theologians say that this is the case, relying on the statements of theologians of the past, approved by the Church, who have studied the problem of the heretical pope, the schismatic pope or the pope who in practice abandons his charge of supreme Pastor. It is not impossible that this hypothesis will one day be confirmed by the Church.” (Ecône, February 24, 1977, Answers to Various Burning Questions)
Archbishop Lefebvre was not a sedevacantist.