I also wonder if they ever actually touched the food after it was fried. All of the transient bacteria on the food would be killed, and if no one touched it directly it would reduce the chance of mold growing - especially on the fries and bun. They would simply dry out.
Same with the meat, it would just dry out, as it appears to have done there. I worked in restaurants for years and it wasn't uncommon to find something that had fallen behind the stove or grill. Cooked meat simply shrivels up a bit and doesn't look that much different after a few days versus a few weeks.
And with a thin burger cooked well-done there would be very little moisture left in it anyway, add to it some salt, and it's already mostly preserved the same as jerky when it is served.
I'm not defending the nutritional value of a Happy Meal, I'm disputing the results of this art project as being meaningful in anyway regarding the healthiness of the Happy Meal.