"Because the Archbishop didn't understand previous teaching."
Give me a break. You hang everything on Religious Liberty as if that explains the break between the SSPX and Rome. One can easily counter with the fact that the Pope did not understand Trent--or the Syllabus of Errors, for that matter. It was the whole idea of revolution which Lefebvre opposed, beginning with a Liturgy which deliberately subverted Catholic dogmas and ending with Assisi which flagrantly affirmed the most radical and heretical form of indifferentism and syncretism ever experienced within the walls of the Catholic Church.
And when you couple all this with the breakdown in discipline on the part of this Pontiff, a breakdown which selectively punished the Archbishop's for his adherence to the ancient faith, yet tolerated every other clerical blasphemy and desecration and corruption humanly conceivable, then you see there is some need for us to draw back and reconsider just how much we owe the Pontiff himself and how much we owe to the Catholic faith. Because the two are at times irreconcilable and no man can serve two masters. Either we must follow the Pope and his revolution--or we must follow the traditional faith.
Religious liberty was the major problem with the Archbishop. Look at his 1985 letter written with de Castro Meyer to the Pope - religious liberty is their main focus, and they say the Pope and bishops will be heretics and depose themselves if they defend the doctrine at the Extraordinary Synod.