Isn't this the case you propose? "the pope can be wrong in his judgment". Certainly the Pope had jurisdiction, there was nothing wrong with the form of the declaration, and the motive was not manifestly inconsistent (schism), nor manifestly incorrect (Msgr. Lefebvre was widely considered to have created a schism by his actions).
Oh, I've never denied the legalities, only the moral validity of the Pope's judgment in the Ecclesia Dei letter. It was erroneous in its evaluation of motives and unjust toward innocent men--and therefore a nullity. But the legal sentence has been widely accepted by the rank and file and has led to a real persecution of innocent men--the priests of SSPX. Fortunately, these good priests soldier on and preach the Gospel notwithstanding.