The installation instructions on this page give a very simple procedure - download, unzip, make a few configuration edits. This is very different from the README page at the Github repository, which says that the extension depends on Semantic MediaWiki. Is it really the case that this extension cannot run without Semantic MediaWiki? If so, this page needs to be updated.
Topic on Extension talk:GitHub
I think you refer to this section of the README. This is actually an example how to set the $egGitHubDefaultRepo parameter using Semantic MediaWiki as an example. I will do a tiny tweak to the README to make this more clear.