In fact, that comment about the donkey is, itself, largely a reaction to the whole of fantasy literature, especially D&D. D&D destroys that medieval economy model, over and over in a hundred ways (permanent Walls of Iron, diamonds from the Plane of Earth) and then just handwaves the consequences with a shrug. And it works fine. They wouldn't feel the need to make that statement if it were not violated by almost every fantasy story ever written. It's a valid perspective, but it's hardly indicative of the sole way to write about magic.