What “early church leaders writings” are you referring to?
Like Irenaeus said:
“To which course many nations of those barbarians who believe in Christ do assent, having salvation written in their hearts by the Spirit, without paper or ink, and, carefully preserving the ancient tradition, believing in one God, the Creator of heaven and earth, and all things therein, by means of Christ Jesus, the Son of God; who, because of His surpassing love towards His creation, condescended to be born of the virgin, He Himself uniting man through Himself to God, and having suffered under Pontius Pilate, and rising again, and having been received up in splendour, shall come in glory, the Saviour of those who are saved, and the Judge of those who are judged, and sending into eternal fire those who transform the truth, and despise His Father and His advent. Those who, in the absence of written documents, have believed this faith, are barbarians, so far as regards our language; but as regards doctrine, manner, and tenor of life, they are, because of faith, very wise indeed; and they do please God, ordering their conversation in all righteousness, chastity, and wisdom. If any one were to preach to these men the inventions of the heretics, speaking to them in their own language, they would at once stop their ears, and flee as far off as possible, not enduring even to listen to the blasphemous address. Thus, by means of that ancient tradition of the apostles, they do not suffer their mind to conceive anything of the [doctrines suggested by the] portentous language of these teachers, among whom neither Church nor doctrine has ever been established”.-Against Heresies (Book III, Chapter 4 verse 2)
It was the Holy Spirit that revealed the truth to their hearts - the "barbarians" who couldn't read, I guess. This "tradition" they were taught was INDEED the gospel of Jesus Christ and the truths of God. The teachings of Christ and the apostles were handed down from church to church for many years verbally, or from notes the disciples took (we don't know that some didn't do that, right?).
When they were ready, the Holy Spirit inspired the writers of scripture to begin writing the teachings down. This was likely due to the growth and the distance between the churches and the availability of teachers. These writings contained the truths, all the truths, God intended for his church for all time, just as he did for the Jews. So the traditions Irenaeus spoke of are all found in the Bible, he even iterates what the truths were that the barbarians would be taught.
The argument today is if the traditions the Catholic Church teaches that are not found in scripture are also inerrant and inspired. That is the disagreement, not whether or not some traditions that were made part of scripture didn't exist prior to the books of the NT being written. Sola scriptura, to me, means that the scriptures are the ultimate authority for doctrines of the faith and, on that, every early "church father" agreed.