Incorrect. “This scroll” is the Book of Revelation.
Wouldn’t you know I’d run into the one person still alive who was there and actually talked with John!
How about these passages:
Deuteronomy 4:2
Do not add to what I commmand you and do not subtract from it, but keep the commands of the Lord your God that I give you.
Proverbs 30:5
Every word of God is flawless; he is a shield to those who take refuge in him.
Do not add to his words, or he will rebuke you and prove you a liar.