Sure someone wrote the lyrics, just not the people who sang them. As for jazz musicians, you're right, in the sense that they played their own improvised solos, they were playing their own music. However, even today many jazz musicians don't compose all that much, prefering to play songs written by others, and express themselves in their solos. As for classical music, I don't know that that's off the topic. I think the point was that it's possible to be doing something musically worthwhile even if you're not playing your own music. However, since imho most of the new pop bands aren't doing something musically worthwhile, the point is sort of moot.