I did not interpret the article as being git should be a primary source of documentation at all. I have a hard time seeing which part of the article implies that. I think he's just pointing out how it's another tool you can use.
A project’s history is its most valuable documentation. (...) The quality of this documentation, however, relies heavily on the diligence of the people involved while writing commit messages.
Requiring such diligence when writing commit messages, while at the same time allowing the same people to write opaque code as shown in the example, would be absurd.
