I was wondering if that might have been part of the 1965 revision for the second edition. At that time JRRT was 73, and had most likely given up on managing to get The Sil published in his lifetime. But my Hammond & Scull "
The Lord of the Rings - A Reader's Companion" from 2005 only notes that originally a part ran "Collect all my notes and papers and take them with you", so the only change from first to second edition seems to have been the insertion of "... , and my diary too, ..." after papers.
Bu-hut.
LoTR was still something of a continuous effort, if with restarts and breaks, and continuously growing (an estimate of JRRT's at an early phase would have meant LoTR would have been ended in about the number of pages now "occupied" by FoTR alone!). The "Sil tradition" was much messier, having been spread over about half a century by the time LoTR was published. Just from memory, he had done two versions of "Lost
Tales", an
Eriol and an
Aelfwine one; broke that off, and started on the two long poems, both unfinished; did a sketch of the mythology; started compiling Annals, ending up with several contradictory versions of those; did several "Quentas" with different suffixes; did several versions of his "Atlantis"; started, but never completed detailed stories on
Túrin and
Tuor; suddenly had characters from LoTR to fit in (
Galadriel!) who had not been part of the original concept, so back writing to do; started a "Copernican revolution" about the Two
Trees and the originally flat earth …. now some of this (to be found in "
Morgoth's Ring" and "The War of the Jewels") post-dates LoTR. Which only made things worse for "
Frodo". But JRRT might very well have had a feeling, even if he, as I would guess, had lost the complete overview of what he had written - never mind finding all of the papers after the odd move of house - that sorting out this jumble would be more of a chore than he was going to be up to. So a bit of a veiled autobiographical touch here with
Bilbo - possible.