That is because he WAS a true Pope.
I think that the point here is that St. Catherine would never adopt the sedevacantist thesis. Feel free to interpret her writings otherwise if you wish, but I think that you will have to distort her intentions in order to do so.
I do not think you are qualified to say what St Catherine would or would not have done re: todays v2 anti-church. Be that as it may I AM NOT A 'SEDE VACANTIST' AND IN FACT HAD NEVER HEARD OF SUCH A THING until a visit to the TCW site.
The evidence is overwhelming that Greg XVII was truely elected Pope in 1958 and was such until 1989.