Fr. Zigrang would be wrong to assent to Bp. Fiorenza's mistaken view concerning the Mosaic Covenant, which could never save men. Certainly on this matter he ought to appeal to the Holy See, since Bp. Fiorenza's opinion appears to contradict the recent declaration "Dominus Iesus".
But there was no reason for him to leave the diocese for the schismatic SSPX - certainly there is nothing wrong with the Novus Ordo which requires a dogmatic rejection of this rite legitimately promulgated and approved by the Apostolic See. And Bp. Fiorenza had given him permission to use the Tridentine for his private Masses, as well.
Wow! And you must consider that a "wide and generous application" of the Indult!
Wrong again. He went to the SSPX because he recognized in them the true Catholic faith.