There's something to be said about reading documentation rather than relying on stackoverflow answers or possibly inaccurate tutorials.

Substitute C, Java, Python, etc for git. You can probably do something with those languages, but you aren't going to get very far without reading some sort of documentation.

The article is about how to get a more fundamental understanding idea on how git works, and this book demonstrates fundamental ideas on how git works. I don't see a problem in this reccomendation. If you want just a cursory knowledge of how to use git to get by, this probably not the right choice, but that's not really what this discussion is about is it?

Unfortunately, it isn't possible to effectively use git without knowing something about the internals. You can do the basics taught more 'by rote', but sooner or later you're going to run into something unexpected, or something complex you need to do and you need to understand the data model in order to have a chance of sorting it out.