Possibly, I didn't get far enough into it to tell.
I thought his son took the note and tried to complete the book with them.

Wikipedia sums it up here

The Silmarillion is a book consisting of a collection of myths[a][T 1] and stories in varying styles by the English writer J. R. R. Tolkien. It was edited, partly written, and published posthumously by his son Christopher Tolkien in 1977, assisted by Guy Gavriel Kay, who became a fantasy author. It tells of Eä, a fictional universe that includes the Blessed Realm of Valinor, the ill-fated region of Beleriand, the island of Númenor, and the continent of Middle-earth, where Tolkien's most popular works—The Hobbit and The Lord of the Rings—are set. After the success of The Hobbit, Tolkien's publisher, Stanley Unwin, requested a sequel, and Tolkien offered a draft of the writings that would later become The Silmarillion. Unwin rejected this proposal, calling the draft obscure and "too Celtic", so Tolkien began working on a new story that eventually became The Lord of the Rings.