Do you not understand that is our central disagreement?
The point is that if scripture is correct, it must be authoritative. If it is not the sole authority, then other authorities must at minimum agree with scripture. Since scripture gives us a positive example of people testing apostles against scripture, then it would be false to say that no one but an apostle can interpret scripture or test an apostle against scripture. And if the apostle had disagreed with scripture, then the "apostle", and not scripture, would be thrown out.
No argument here, with the single caveat that "agree" can be rather furry all on it's own.
Since scripture gives us a positive example of people testing apostles against scripture, then it would be false to say that no one but an apostle can interpret scripture or test an apostle against scripture.
That is similarly fuzzy in that the standards of that test are unknown except those in judgement reckoned the apostle to be in conformity to The Old Testament. The New Testament had yet to establish it's bona fides at the time.
As an aside, bear in mind there were plenty of Jews who would NOT agree the apostle was in conformity with the Scriptures, so the standard being used was by no means universal.
I am unaware of any contention that only an "apostle can interpret scripture or test an apostle against scripture" but similarly there is no Holy Writ saying just anyone can interpret Scripture or test apostles.
And if the apostle had disagreed with scripture, then the "apostle", and not scripture, would be thrown out.
Again, the Old Testament had been long established and needed no validation. The same can not be said for any letter floating around claiming to be from an apostle of the Lord.