There is no presumption of validity when canon law is in contradiction with a papal letter. Papal law trumps a papal opinion expressed in a letter.
The canon law was not in contradiction.
[The consecrations] were performed expressly against the will of the Pope with a formally schismatic act according to the norm of Canon 751 , he [Archbishop Lefebvre] having openly refused submission to the Supreme Pontiff and communion with the members of the Church subject to him. [. . .] he cannot even apply Canon 1323, any relevant matter foreseen by him not being verified in this case, since even the alleged "necessity" was deliberately created by Archbishop Lefebvre in order to preserve a posture of separation from the Catholic Church, notwithstanding the offers of communion and the concessions made by the Holy Father John Paul II. (L 'Osservatore Romano, "Notice", 3 July 1988)
As the Canon Law says:
Can. 1325 Ignorance which is crass or supine or affected can never be taken into account when applying the provisions of cann. 1323 and 1324. Likewise, drunkenness or other mental disturbances cannot be taken into account if these have been deliberately sought so as to commit the offence or to excuse it; nor can passion which has been deliberately stimulated or nourished.