My point in the sentence you ask about is you can not claim both the first clause AND the second clause without the third clause being true.
Questions:
Was metmom claiming a special charism of assured infallibility, as like the pope (which your response seems to charge her with claiming), or was she expressing a belief, that the Scriptures are inerrant/infallible based upon its statements and attestation?
And are you saying that no one can have Scriptural certitude (and thus believe and speak the same) except by confidence in the magisterium of Rome when it speaks infallibly?
Thanks