Even a cursory reading of the Old Testament shows that God speaks through prophets orally, not just in writing. "Thus sayeth the Lord" was not referring to written documents when uttered by the Prophets, who spoke as the Spirit led them (see 2 Peter 1:19-21, quoted above).
While certainly not every claim of "thus sayeth the Lord" should be accepted at face value, the very fact that the Scriptures themselves speak of such situations, and show the people obeying the spoken commands, demonstrates along with the other verses cited, that Sola Scriptura was not a principle during Biblical times, whether Old or New Testament.