>Is the word "myth" here being used in the sense of being true but connected to storytelling nevertheless? Because the Great Pacific Garbage Patch really exists

From TFA, which pretty much says the same thing with different words:

>In fact, the Great Pacific Garbage Patch was barely visible, since it comprised mostly micro-garbage. It can’t be scanned by satellites, or scoped out on Google Earth. You could be sailing right through the gyre, as many have observed, and never notice that you’re in the middle of a death-shaped noxious vortex. The patch is such a wishy-washy phenomenon, with wishy-washy impacts, that its extent can’t be described with any certainty. (“No scientifically sound estimates exist for the size or mass,” notes the National Oceanic and Atmospheric Administration.)

What doesn't exist and is a myth is the "Great Pacific Garbage Patch" as usually described in the media (the miles of visible floatsam, actual whole garbage thrown in, etc).