When the Bible uses the phrase, "God's words," it does not refer only to the Bible. The Bible refers to itself as "the Scriptures," that is, the written word of God. "God's words" include the Scriptures, and also the authoritative teaching of the Church which preceded and is the basis of the Scriptures. So in Proverbs 30:6, the Lord commands us to add nothing to the teachings of the Church, including both its teaching through the Bible and its oral, authoritative teachings throughout the centuries.
However, many people nullify God's word in favor of merely human traditions (Mk 7:13). For example, they take pop-psychology and the propaganda of secular humanist institutions as the gospel truth, while denying the truth of the gospel. We nullify God's word by adding to it in giving unquestioned authority to those ideas which the media chooses to present as infallible.
At the same time, we are arrogant and confused enough to subtract from God's word. We are tempted to take less and less of it authoritatively, as we ignore the Church and water down the Bible. In the last few verses of the Bible, the Lord warns us: "If anyone adds to these words, God will visit him with all the plagues described herein! If anyone takes from the words of this prophetic book, God will take away his share in the tree of life and the holy city described here!" (Rv 22:18-19)