Can `add-apt` be used for Github repos?
Debian package repositories (which are what PPAs are, but with specific packages in them for Ubuntu releases, and not Debian itself) are not the same as git repos. You can't use a git repo for an apt repository.
The main reason for this is because git uses a special server designed to work with source code, and not binary packages. An apt repository server can be set up with a package manager backend and almost any web server, and is specifically designed to work with binary packages.