Thank you for your interest and your response, Ray.
Yes, the variety of definitions of the word ‘medium’ make it difficult to understand the phrase. Even Marshall used the word differently when, as in the title of the book, he called media ‘extensions of man’. There I believe he is speaking of technologies (artefacts) themselves and not the environments which accompany them.
Laws of Media: The New Science was written to be the revised/extended edition of Understanding Media: The Extensions of man.
Is the artefact the message? In the context of the phrase ‘the medium is the message’, where medium=environment, I would say it is not. The artefact, or technology, is part of the content of the environment.