That Scripture passage, contrary to how you state it, proves to us that Holy Scripture's purpose is to be THE authority for our faith.
Matthew 28: 18* g Then Jesus approached and said to them, “All power in heaven and on earth has been given to me. 19h Go, therefore,* and make disciples of all nations, baptizing them in the name of the Father, and of the Son, and of the holy Spirit, 20i teaching them to observe all that I have commanded you.* And behold, I am with you always, until the end of the age.”
Jesus never told anyone to write anything down. Those who did were inspired to. God did not dictate, as He did to Moses. Remember the opening words of Luke? Jesus told them to teach. Not spread books. The Church decided to write things down and disseminate them later. Much later. 20 years later. Were the teachings invalid because they had no book?
That is why John said what he did that not everything Jesus did was or could be written down, but what has been written is so that we may KNOW and BELIEVE "that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name". Any extraneous writings or traditions that subtract from that assurance can and should be denied and counted as false.
That is not what John said.
John 28: 24It is this disciple who testifies to these things and has written them,* and we know that his testimony is true.n 25There are also many other things that Jesus did, but if these were to be described individually, I do not think the whole world would contain the books that would be written.o
John actually said that he wrote this down and his testimony is true. There were perhaps 80 Gospels written, with many variants of each one. If they were dictated and contained all that we are required, then why did God not dictate a single Gospel? Why are all four different?
1. Matthew: Christ presented as the King of Israel. "Behold THY KING" (Zech.9:9).
2. Mark: Christ is presented as The Servant of Jehovah. "Behold...MY SERVANT". (Zech. 3:8).
3. Christ is presented as The Perfect Son of Man. "Behold THE MAN". (Zech. 6:12,13).
4. Christ is presented as The Eternal God. "Behold YOUR GOD." (Isa. 40:9).
Matthew establishes His Royal Rights to the Throne of David. Mark gives no genealogy. That would not be an issue with a Servant. Luke establishes His full human and Natural Lineage. All the way back to Adam. John also presents no genealogy. He is traced back "from everlasting" The Eternal God.
There are four gospels because they represent the four-fold picture of Jesus Christ's earthly life and ministry. Perfectly God and perfectly Man.
It most certainly IS what John said.
John 20:30-31
Jesus performed many other signs in the presence of his disciples, which are not recorded in this book. But these are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name.