What do you mean by “Christianity” sees it as referring to “both” the Book of Revelation and the Bible as a whole? Where can I read that this is how “Christianity” sees it?
Mormons do not believe that the Book of Mormon is an addition to the Bible. It is used
in addition to the Bible, but it is not added
to the Bible.
There are earlier scriptural verses that make similar claims about not to “add to or take away” from “this book”. If we are consistent in interpreting this as referring to the Bible, then this would make all later scriptural texts invalid, as they are “additions” to what was already written.
Here is an excerpt from “Catholic Library Quick Questions (1992)”, a response to the question of whether Rev 22:18 invalidates Sacred Tradition:
"That conclusion might be possible if John’s phrase “this book” meant “the Bible,” but it doesn’t. It’s a common mistake of Evangelicals and Fundamentalists to assume that John was speaking here of the Bible as we know it–all 73 books (seven less in Protestant versions), from Genesis to Revelation, bound between two covers.
John wrote Revelation before the year 100, so he could not have had the Bible in mind when he penned this warning, because the Bible as we know it (and as many Protestants think he meant it) would not exist in its present form for three centuries.
The Catholic Church defined the canons of the Old and New Testaments at the councils of Hippo (393) and Carthage (397). Before that time Christians weren’t certain exactly which books belonged in the canon because the Church hadn’t yet made a definitive decision on the issue.
…
There’s another reason Revelation 22:18-19 doesn’t disprove the Catholic doctrine of Tradition. Virtually the same warning is given in Deuteronomy 4:2. If we apply there the same principle that you want to apply in Revelation 22, we have a dilemma, because God would have prohibited the adding of anything to his statutes and decrees as found up to and including the book of Deuteronomy. If that were the case, all subsequent books of the Bible, including the book of Revelation itself, would be proscribed because they were added to the Pentateuch. That means Jeremiah and Ezekiel and Paul and John and all the writers of later books would have the aforementioned dreaded plagues “added unto them” because they added to what was already there.
So what was John really warning us about in Revelation 22? Simple. He had written the book of Revelation as a prophetic document for the edification and guidance of the Church, and he didn’t want it tampered with–nothing added, nothing subtracted. He knew that some knucklehead in a later generation might decide he could improve on the message, or, like the Jehovah’s Witnesses, twist it to better suit his personal theology. Revelation 22:18-19 is essentially a first-century copyright, designed to discourage people from altering the work. "
newadvent.org/library/almanac_thisrock92.htm