I don’t think, whether he used the plural or the singular, that he was speaking of dress. But as I said, I never understood until thinking I’m beginning to get it. I bought one of his books 25 years ago and couldn’t understand the table of contents.
The medium is the message (one layman’s understanding):
I believe he was referring to the context that one appears in, the surroundings, the clothes, the projected attitude...
that your medium (that you are immersed in, what you paint your story with) is your message.
Dress and act as a good doctor...says, “This is a good doctor” and you don’t have to say it yourself.
Oil paint is a medium for expressing art on canvass. Watercolor is another medium. The medium is the message...
means that your choices of style around you tell the customer what the message is before you open your mouth to explain it.