[The Squatter of Amon Rûdh]

The idea that this event was added to the Red Book of Westmarch at a later date is entirely consistent with Tolkien's mythical history of The Lord of the Rings as a translation of a Fourth-Age document. Ancient documents often contain additions by later generations, particularly historical chronicles. I would guess that one of the Fairbairns is supposed to have inserted the date for the sake of completeness after Gimli and Legolas had made their departure.