The Solemn Magisterium is the final say on any matter of doctrine and may never be lawfully opposed or dissented.
I can prove any dogma I want if I get to say who is and is not a member of the Solemn Magisterium.
For instance, suppose I say Popes Eugenio IV and Boniface VIII are heretics. What happens to the rigid interpretation of EENS?
You don't get to give liberal post-Vatican II popes who kiss Korans the authority to contradict those who came before.
That's not about "you deciding" - None of us decided to make the post-Vatican II popes act in manifestly heretical ways.
Define 'Manifest Heretic' Then demonstrate how the Popes acted in such a fashion.
Pax makes the point that, authoritatively, he has as much power to denounce ANY pope as the sedevacantist does to denounce a V2 pope.
(At least, that is what I think he is trying to point out.)