The problem was that they got their myth wrong in the first place.
First of all, they arbitrarily decided that the splinters had to be "lethal," which is a complete fiction, and then they arbitrarily labeled it a "pirate" myth instead of a general fact of naval warfare.
When of course it didn't matter whether splinters were lethal so long as they caused casualties, and pirates hardly used their guns at all, much less roundshot against hulls.