I find that hard to believe, because Jackson's LOTR trilogy (which again, needed FAR less "supplemental" material to bring it up to nine hours) had several major elements that were made up for films and NOT based on material from Tolkien's books. For example, the half-elf Arwen Undómiel is change from a minor character to a major character and the movies invent all sorts of new scenes and actions for her that weren't in the books versions of LOTR or The Hobbit. Second, Jackson does NOT have the rights to the other books set in Middle Earth (The Silmarillion, The Children of Húrin, etc.) so he couldn't even use material from the posthumous Tolkien books without Christopher Tolkien suing.
Actually, in one of Tolkien’s supplemental books, he wrote a fairly substantial poem laying out Aragorn’s and Arwen’s love for each other, so it’s not really an insubstantial part of the LOTR world.
It was also done for the most practical reasons. If he hasn’t done it, it wouldn’t have attracted half the box office it did, and there would be no three hobbit movies to enjoy.
There aren't many scenes invented for Arwen. What PJ mostly did was transfer the actions of others to her, notably Glorfindel, who otherwise is a total McGuffin.
Also to pick an elven nit, Arwen was 3/4 elf. Her mother was Celebrian, the daughter of Galadriel and Celeborn.
Her father was Elrond the Half-Elven.