See more at the Statistics Forum (of course).
If none of the co-authors was the plagiarist, then the authors still used someone else's writing in their paper. That's plagiarism, in many people's use of the term. At best they didn't know that the mystery person was in fact plagiarising, but by using the material they took on responsibility for it and so _they_ were plagiarising.
Of course, there's a spectrum of practices. I heard of a leading U.S. statistician who delegates some of his book reviews to smart graduate students. The (very grateful) ex-student who told me said, in effect, it's just his way of working. He makes the deal evident beforehand and makes it up to you in other ways by superb mentoring. Papers too, in other cases?