Because if salvation is available to people who are not under the authority of the Roman Catholic Church organization, then the "Church" is not the Roman Catholic organization, but the entire Body of believers in Christ, who are members of any and all organizations that subscribe to the teachings of Jesus and are saved, not by some Church, but by the grace of God through faith in Jesus Christ.
So, regardless of whether or not Peter was the first pope, if Salvation is available outside the Catholic Church, then it is irrelevant who is, or is not, Pope.
So, polemikos, what is it? I claim to be a member of the Body of Christ. I claim to believe in Christ through faith. I am not now nor have I ever been a member of the Catholic Church. Is it possible that I am saved?
I can answer that. You're Invincibly Ignorant. You'll be fine. Welcome to my world. :-)