“Paul, an apostle, (not of men, neither BY MAN, but BY JESUS CHRIST, and GOD THE FATHER, who raised Him from the dead;). Galatians 1:1.
Matthias was an apostle by having the lot fall to him, even Judas was called an apostle, it seems to me that Paul took it upon himself to say he was an apostle. Doesn't seem right.
That would not make his writing less, just that he is a self appointed apostle..