It is impossible that bishops teach heresy with a moral unanimity. The authentic magisterium is infallible when a doctrine is determined irrevocably, being defined properly speaking. Now, if the doctrine is defined, one can no longer say that it is merely an error, because it is taught as de fide. If an error is taught as de fide, it becomes a heresy, not a mere error.
It is impossible to say that Vatican II was infallible, for this very reason. The SSPX is right on that point. But the sedevacantists are right when they answer that one has to obey the Pope, whereas the SSPX does not obey Francis. If the SSPX does not obey Francis, it means it does not ackwnoledge him.