> this looks to be an elongation of the external occipital protuberance

Good call, that's exactly what they are. They buried all the actual science at the end of the article.

"The effect — known as enlarged external occipital protuberance — used to be so uncommon, Sayers said, that one of its early observers, toward the end of the 19th century, objected to its title, arguing that there was no real protrusion."