It'sIt’s not eyewitness testimony, it's a literary flourish
The most likely perpetrators of this are either Samwise Gamgee, its original author, or J.R.R. Tolkien its translator into English. However, it is possible that this flourish was added during the transcription from the Red Book to the Thain's book or in the final transcription back to the manuscript stored in Great Smials that served as Tolkien's reference. If anyone has access to this manuscript and can speak Westron (I can't), they will be able to tell you if the flourish is in that document.
I lean towards Tolkien; it has been argued1 that Tolkien's translations of Beowulf were a work of metafiction rather than a mechanical translation (insofar as anything beyond Google translate can be a mechanical translation). In the case of both the Red Book and Beowulf, while both purport to be a history of real events, from this distance both are legendary which is to say it impossible to determine how much if any is factual and how much is the author recording myth and legend as fact.2
1Vladimir Brljak. "The Books of Lost Tales: Tolkien as Metafictionist." Tolkien Studies 7 (2010): 1-34. https://doi.org/10.1353/tks.0.0079 (accessed September 23, 2019).
2Yes, I know that the LotR is Tolkien's original work but it's more fun to pretend it isn't just like Tolkien did.