"Many designers or agencies might scoff at the notion of a “non-designer” being able to learn and understand the principles to help them arrive at a design."

They would, except that most of them were once non-designers who learned the principles of design. That is how you become a designer.

Or maybe he means non-designers can learn how to design by reading a book? Yeah, I guess designers would scoff at that - reading helps, but you really have to practice to be any good.