The underlying model of git is not too hard to understand, and is actually pretty elegant. There are a number of good explanations online, including the one you linked.

The git tool itself, though, often operates at a much higher level of abstraction. I think that's more where the reputation of being hard to learn comes from. For instance, you'd probably need to write a paragraph or two to explain what "git checkout <branch>" does in terms of the actual object store. Add to that that the CLI is poorly designed - many commands have misleading names, a given command will do completely different things depending on the flags, there's a lot of implicit / counterintuitive behavior, and so on. See https://stevelosh.com/blog/2013/04/git-koans/ for some funny examples.