Canon Law IS the law of the Supreme Pontiff. Did you think it existed on its own? It is the Pope speaking canonically. Not only this, but it trumps any of his pontifical letters which would lack the same sort of official status.It does not, hwoever, trump him. He can trump it whenever he wishes, he can change it at will, and when he issued the decree Ecclesia Dei, he did just that to whatever extent the earlier written Canon law disagreed. I would note, that IMHO, there is no conflict, the canon law arguments for Lefebvre are hollow and false, but it really doesnt matter. The Pope trumps the canon law whenever he wishes. Vatican I:
8. Since the Roman pontiff, by the divine right of the apostolic primacy, governs the whole church, we likewise teach and declare that he is the supreme judge of the faithful [52] , and that in all cases which fall under ecclesiastical jurisdiction recourse may be had to his judgment [53] . The sentence of the apostolic see (than which there is no higher authority) is not subject to revision by anyone, nor may anyone lawfully pass judgment thereupon [54] . And so they stray from the genuine path of truth who maintain that it is lawful to appeal from the judgments of the Roman pontiffs to an ecumenical council as if this were an authority superior to the Roman pontiff.To the extent you claim that Canon law somehow constrains the actions of the Pope, and his judgment excommunicating Lefebvre, over his own personal power, you are a heretic.