Mostly true. In both cases (the Torah or Bible), you need the text to lay out the landscape. In the case of the Bible, no one made a rule that you couldn’t change the wording, edit it, or even add onto it. A fair amount of the Old Testament comes from Torah material. One might imagine some hostility existing when a second crowd came up and added the New Testament onto the back of this document. In the existing culture, there isn’t any rule to hinder even the existent or addition of a 3rd testament onto the back of the first two.
In the case of the Koran, nothing can be edited, or added. If some angel did appear to one million Muslims today, giving out 40 additional pages of guidance....well, because of the rules in place, the forty pages could not be added because there weren’t going to be any other visitations or additions.
Rev 22
18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.