Actually, in the Greek text (consult any good commentary on the original text) Jesus said, "Whatsoever you shall loose on earth SHALL HAVE BEEN LOOSED in heaven, etc..." Jesus was not giving them the authority to make up their own doctrine, rather he was stating that their inspired utterances would echo what had already been determined in the mind of God.
Doesn't make a difference. There was still no "New Testament" to be preached from, and it would be preposterous to assume that everything they preached - every miracle recounted - every parable told - ended up in the canon.
Everything IN the canon is truth, but the story of salvation continues to unfold, otherwise, how could anyone even begin to defend Luther, who used no Scriptural citation in his infamous "95 Theses" - based on his interpretation of tradition and with no support from Scripture.