There are some who will appeal to +Lefebvre and say that he thought the sedevantist proposition was a real possibility to hold to. There is one specific quote used by Sede's to lure trads into their disorder:
"I do not say that the Pope is not the Pope, but I do not say that you cannot say the Pope is not the Pope."
Now this sounds rather damning. However, the former SSPX seminarian (expelled by Lefebvre before his ordination gives context to the quote:
"(Said at table to fellow clergy, to laughter, for the phrase in French sounds like a tongue twister.)"
In other words this is an informal, off the cuff discussion and seemingly not very serious. Furthermore it should become obvious that when Fr. Cekeda was expelled as a seminarian (along with others) he was expelled for holding to the sedevacantist position.
+Lefebvre is a very complicated character in Church history. Some people like to compare him with St. Athanasius, but I think this is more piety than reality. It is, however, more interesting to look at him along the lines of Friar Savonarola during the Renaissance.