Says who? Peter seems to believe that all the writings of the apostles were considered scripture.
2 Peter 3:15b. As also our beloved brother Paul wrote to you, according to the wisdom given him, [16] as in all his letters, speaking concerning these matters, in which some things are hard to understand, which those who are untaught and unstable twist to their own destruction, as they do also THE OTHER SCRIPTURES. [17] You then, beloved ones, being forewarned, watch lest you also fall from your own steadfastness, being let away with the delusion of the lawless.
What? Where does Peter say he’s referring to the Acts of the Apostles? Or to any specific book of the New Testament? And obviously Peter CAN’T be referring to those books of the NT that hadn’t been written yet!
BTW: Luke, not Paul, wrote the Acts of the Apostles.