True to an extent, but ultimately Marshall McLuhan overlooks the role that content has to play.

For example the protestant reformation changed the entire political structure of the west, and the way that people thought of each other. The key point of the protestant reformation was that every man could go into his closet and find God for himself. This allowed people to go into their closets and find a God that didn't support the current status quo (that is to say the divine right of kings, and the social order it supported).

McLuhan's analysis would've been something like the Gutenburg press supported literacy and free thought by removing church control on the dissemination of information.

Sure, the Gutenburg press had a role to play in changing society and the way it interacts in and of itself, but if they had printed of 1,000,000 copies of Beowulf then not much would've changed. The only literate people were aristocrats and clergy anyways. But they didn't print off Beowulf. They printed the bible. Now that everyone had access to their own copy of it (instead of relying on the parish copy) they had the ability to read it and interpret it themselves.

Example 2: The Bolshevik Revolution

Tzarist Russia thought of their peasantry and proletariat as less than dogs. Leo Tolstoy's War and Peace spoke of them as real people, with real personality and hardships. It's this writer's belief that this work helped the proletariat realize that they're people, eventually resulting in the revolution. It wasn't the invention of books or the printing press that caused it, it was the specific work.

This isn't to say that the idea of the medium is the message is baseless, just that it's only part of the whole picture