Commentary By Fr Luc
One really has to penetrate the mind of Pius XI in this matter of his Motu Proprio. He effectively gave Archbishop Thuc a mission.
Here, I can only write as I, a man of Faith, see it.
The 19th century saw the Church blessed with some very holy Popes. One such was Pius XI. In such calamitous times Our Lord does not abandon His Vicar. He does appear to him and instruct him. Such occurrences are well documented with Pius XII.
Is it beyond belief that the Church at the time of Pius XI was well infiltrated and on the verge of destruction? Remember this is several decades after Leo XIII brought in the Leonine Prayers after Mass.
I believe Pius XI knew that a Council was on the door step that would bring in Liturgical reforms that would invalidate the Sacrament of Episcopal Consecration; which over a span of 50 years was to leave the Church Priestless.
Archbishop Thuc's mission was to leave behind validly consecrated Bishops.
This he did. The rest is up to Our Lord to use those open to His Grace!