Having not read the article I then guess this was an instrument for the wealthy who could read? Or, would poor buy something they cannot read? Maybe if it was a common saying and then they have the meaning memorized.
Or its like a shampoo bottle and it's written there because the carver felt like it.
You guessed correctly, per the article: "It would have been like a diamond today, a crème de la crème luxury item. Others likely had lice combs too, but made of wood that would have decayed"