It’s possible both to reject strict sedeplenism in the sense that ladislaus describes, to say that the Church might yet declare one or more of these popes are in fact antipopes, AND yet say that to be a Sedevacantist and to overtly reject communion with these men while they are recognized by the Catholic world as pope sin schismatic.
I’m not sure if that’s what lefebvre thought, or even if that’s correct, but I see no logical inconsistency on it