The myth is where the meaning comes from, but once it becomes a word in the general language so that people can understand what you mean by it even if they don't know the original myth, then it is more than just a "reference". Without the historical Mr. Boycott, there wouldn't be a word "boycott" in the dictionary, but that doesn't make "boycott" just a reference to his original story; almost no one who uses the word knows who he was any more.
But the main point I was trying to make is not about exactly how
Cassandra should be presented in the dictionary. It's that it is misleading to use the word "meaning" when referring to a name's etymological origin.