I read the book, Sedevacantism: A False Solution to a Real Problem, some years ago. I remember thinking at the time that the author made a lot of good points but most of them were not even about sedevacantism. Frankly, I don't remember the case described above, but it would seem to me that if Archbishop Thuc has anything to do with the issue, his consecrations would tend to indicate that God's Providence used the archbishop to keep the Church alive while the Conciliar church was out there invalidating the line of succession of the bishops from the Apostles by invalidating the Conciliar Rites of Orders--especially in regards to the consecration of bishops, which would support sedevacantism rather than refute it.
However, I do not think Archbishop Thuc's actions is evidence either for or against sedevacantism. The evidence for sedevacantism can be summed up in the publicly professed heresies of Jorge Bergoglio.