Clearly, McLuhan was talking out of his ass.
(Here is what is true about the medium and the message:
The medium influences, sometimes profoundly, how the message is perceived.
The message can't be separated from the medium that conveys it.
Some types of media and some kinds of messages mix well. Some don't.
Them are some important insights. But, especially for professional communicators, it's important to distinguish medium from message—communication vehicle from communication content.
So next time one of these technobabblers mindlessly says, "The medium is the message," let's retort, "Then why do we have different words for them?")