Good find. I did some more research on John DeFrancis and found his writings on the topic. Pg 32 discusses the "love letter" being part of a semi-ritualized party game. [1]
John DeFrancis was an amazing thoughtful and insightful linguist and was, as you can see from this example, very happy to look into what he considered bizarre claims (which he sometimes discovered were true). I'm a huge fan of his work.
Very impressive indeed; I only clicked the 'Visible Speech' link thanks to reading your comment; the diagram illustrating the lineage / spread of the the 'Yukaghir Love Letter' is worth the click alone.