You are referring to the dogmatic decree:
Pope Boniface VIII, Unam Sanctam, Nov. 18, 1302, ex cathedra:
“With Faith urging us we are forced to believe and to hold the one, holy, Catholic Church and that, apostolic, and we firmly believe and simply confess this Church outside of which there is no salvation nor remission of sin… Furthermore, we declare, say, define, and proclaim to every human creature that they by absolute necessity for salvation are entirely subject to the Roman Pontiff.”
Sedevacantes do not deny that "every human creature ... by absolute necessity for salvation are entirely subject to the Roman Pontiff", as ALL the non-Catholics do. They simply do not believe B16 is a real pope. BIG DIFFERENCE. Sedevacantes recognize the office of the pope.
All that said, you correctly pointed to one reason why I said "I won't go there" (sedevacantesism), and "I'll let God sort that out" (God would make it more obvious if the popes were false.
When Catholics once again return to the practice of the faith, they will have good shepherds that lead. The popes we have had since John XXIII are exactly what the people wanted, shepherds that let them do what ever they want. You should feel good about them, they tell you that you can be saved right where you are, along with the Kennedy's, Nancy Pelosi, and Billy Graham, the 33rd degree Mason.