The Archbishop knew the principles envolved. He knew that there may come a time when he would have to say that the Roman See was vacant due to manifest heresy of the papal claimant. He stated this explicitly in 1986: http://strobertbellarmine.net/angeluslefebvre.html
The problem is that the common teaching that a manifest heretic is not a member of the Catholic Church and thus cannot be Her head was widely held by the SSPX faithful and “hierarchy” before +ABL died. Obviously not everyone held this, but it is certain that +ABL did. When the heresy of JPII, Ratzinger and especially Bergoglio was so blatant that one could not escape from the obvious conclusion that these men could not possibly be popes, the narrative changed to one that did not exclude heretics from being members of the Catholic Church nor from being the pope.
But remember, +ABL dealt face to face with the conciliar popes Paul VI and JP2 on many occasions. If there was anyone who could have said with the same certainty that many sedes seem to have, then +ABL was that one - instead, he expelled the sedes, "the nine", from the SSPX. While he may have tolerated it for a time, he expelled the nine, showing that he disagreed and was not sympathetic toward sedesim whilst he lived.
There is another aspect to consider. Namely, consider that the pope believes it to be a teaching of the church, just the same as NOers and the sedes, that as pope, whatever he says or preaches in union with the bishops is infallible. With that in mind and if you have faith in that teaching, neither he nor the other conciliar popes can possibly be manifest heretics, instead, what sedes believe to be heresy, is in fact, without the possibility of error, which makes the sedes completely wrong - per their idea of what the Church teaches regarding the infallibility of the pope.