Defilers and Preservers: The Wizards of Athas was the second last Dark Sun product to be published in the 2E line, and possibly the most problematic. I take anything written in that product with a pound of salt.
Based on every other DS product that discusses magic, it seems implausible that a wizard would not know the difference between defiling and preserving. Even then, the difference is not so great that a commoner would not think a defiler and preserver is the same thing. The only casters of clean magic, also introduced for the first time in Defilers and Preservers, are the Gray, Black and Cerulean casters, all of which overturn the entire premise of the setting.