Yes, a co-writing credit can mean anything from "wrote the song" to "the producer liked some of their suggestions for the second verse" to "it's in the performer's contract that they get a cut of the publishing rights." And ghostwriting, or ghostcontributing, abounds.

In a not-very-related example, one of the differences between Charlie Chaplin and Buster Keaton is that when Chaplin instructed other people to fulfill his general vision on an aspect of the filmmaking, and they went ahead and did the work, Chaplin gave himself the credit; whereas Keaton would hire a buddy to just stand behind the camera and tell Keaton if the shot went like Keaton wanted it, and then give the buddy directing credit because he thought hogging the credit was pompous.