I think his point is dont try too hard to control how people perceive you. (Not whether suits are appropriate attire.)

The rule of thumb is, "other people aren't thinking about you, and if they are, they are wondering what you are thinking about them."

That being said, just like great design influences how people perceive your product, dressing right for the part is equally important.

I read a case study where chiropractors make more money when they wear a lab coat, stethescope, and all their employees wear scrubs.

Your "packaging" has to match your customers expectations

and the story going on in their head