The article could be more helpful by naming hagfishes from the start.

"As for the earliest vertebrates, Mayashita and his co-authors argue that another group of fish, characterized by bony armor in their skin, are closer to the mark. Like lampreys, they have no jaws, and appear to have given rise to other boneless vertebrates."

They never name the other group of fish, but the CMS is smart enough to link to hagfish slime. If you're curious about the origins of vertebrates: https://en.m.wikipedia.org/wiki/Agnatha