It's like that saying about mushrooms: "All mushrooms are edible. It's just that some mushrooms you only get to eat once."
Anyway, edible normally means "safe to eat," not just "possible to eat." (As you are no doubt aware). IIRC, Elmer's glue is considered safe to eat though not necessarily appetising.
I believe that the stereotypical "craft food" is actually paste, which is often based on starches like corn or wheat. Children are very likely to put paste in their mouths and try eating it, because it is indeed based on food products.
I've frankly never been in a school that provided a lot of paste, and the switch to Elmer's glue may have been a strategy to stop kids from consuming the food-based stuff. However, I was in a summer science course where we crafted "Oobleck" which is also sort of "edible" if you like eating clay that's been squeezed between the filthy little hands of 8-year-old boys.
For a product that is only advertising one thing in a photo, e.g. an ice cream cone with ice cream on a package of just cones, I don't think there are any restrictions on what the "ice cream" can be made of. (It's probably mashed potatoes, though.)
But also many post 2000 claims that it was all actually real food because of various "truth in advertising" regulations around the world.
The linked Canadian McDonald's video would be one example.
