If you think McLuhan is full of hooey and overrated (a valid but imo wrong opinion) then you probably won't like this post. That's OK, too.
McLuhan's dictum is famously about media through which messages are conveyed. His notion of hot vs cold media, his ideas about the use of advertising to control desire, and the deliberate separation of effects on male vs female, all of that was about technology. But there's a sense in which "medium" can be replaced with "speaker."
The speaker is the message. Who says a thing is as, or more, important than the thing that is said. On the face of it, that ought to be wrong. Messages can have truth values assigned and determined, and in many situations we try hard to blind ourselves to the speaker in order to value the message. Scientific experiments famously use double-blind procedures so that the results are influenced as little as possible by knowledge of the participants and experimenters.
But think of it this way: imagine a text written by a black woman that I would speak. The text would have a different meaning coming from me - with my appearance, background, and life-experience - than it would from a black woman. Same words, different message. We are, the aphorism goes, each the experts on our selves. For all that there are shared values, similar experiences, and many other commonalities we each remain the unique medium best suited to carrying our unique message.
But wait, I hear there are people who get paid to do just this thing - impersonate others - actors. My current thinking is that yes, this is a case of medium/message that we accept within a pre-agreed constrained fiction. We know someone is performing as another, and accept that suspension of reality. Even within the realm of acting, medium/message constraints apply. Othello is black, Shylock is Jewish, Orsino is cis-male. These constraints in fact form part of the bases on which the storytelling is built, and why cross-casting is so difficult and rare. It's also part of why things like blackface are offensive - even within the fiction of theater, blackface imposes a further fiction that implicates racist attitudes in a way that having a black actor might not. Either way, the person (medium) is the message.
Thinking about acting leads me to think about staging: a black actor playing a black character who is, say, a butler or driver for a white person is a message as well. It's why minorities complain about the roles they are given in television and movies - the presence of a person in a given position becomes a message. Put a person of a particular appearance (skin color, gender, name, accent) in a certain position and they become a message even if they never speak a word. This is true in life, as well as art. Think about how many photo ops are carefully staged to have (or notably lack) representation of different persons, different types of people.
A person who works prominently for a particular company also conveys a message even if they aren't an official spokesperson for that company. I think this is particularly true in our society where we so strongly identify our selves with our work. What are you? A plumber, a programmer. True. Incomplete but true and that truth becomes part of the medium which is still the message.
(I have more thoughts on this, but they're even farther off the metaphorical cliff. Perhaps more later.)