the Roman Pontiff and all the bishops, the priests and the people conjoined with him in the unity of faith and communion fell into heresy when they approved and professed the definitions of the Ecuмenical Vatican Council. Therefore they deny also the indefectibility of the Church and blasphemously declare that it has perished throughout the world and that its visible Head and the bishops have erred. "
No, it’s the sedes who believe the Church has perished throughout the world.
I believe the point is that PPIX is condemning the idea of saying the pope, all the bishops and people would fall into heresy by approving the definitions of an Ecuмenical Council (V1), and that to say so amounts to, or admits that the Church has defected, which is heresy.
The sedes understand this to mean the conciliar popes were/are not popes because PPIX essentially is saying if that were to ever happen, then the Church will have defected.
What is being missed here is the fact that V2 never having defined anything at all, never had even one definition to profess. Had V2 defined one of V2's heretical doctrines, then the sedes would indeed have a legitimate argument and the condemnation of PPIX would apply, but it doesn't apply to V2 or the idea of a schismatic SSPX.