Like, there’s lots of information about Bilbo Baggins in Lotr, that doesn’t mean it was written in the third age of Middle Earth homie

The conceit of the LOTR appendices is that Lord of the Rings, as published in English, is really just the Red Book that Bilbo writes at the end. Dr. Tolkien merely found the manuscript somewhere and has graciously translated it from Third Age common language into English for the benefit of us modern people.