The even vaguer construction "an Arlo Guthrie song" probably means one that he has often performed rather than one he wrote. And "an X movie" almost always means one that X acted in or (possibly) that X directed -- almost never one that X wrote the script for.

From my very marginal position, this looks like a lot of foolishness. It shows that people take performers far too seriously, probably because they are very visible & thus easy to identify with.