Actually the main complaint of Archbishop Lefebvre had less to do with the legitimacy of John the XXIII or Paul the VI and more to do with what they were doing to the Sacraments. Now as for Archbishop Thuc he was for all intents and purposes a man without a country and likewise did not question the legitmacy of either John the XXIII and or Paul the VI. He , unlike Archbishop Lefebvre signed the documents of Vatican Council II . He only began to question them when he witnessed the destruction of Faith in the aftermath of the changes made by Paul the VI to the Sacraments. That is the Crux of the matter IMO.