I don’t believe the GitHub repo was intended to contain all the official extensions. There’s an explanatory post here about which types of extensions can be found where (although the Inform7.com website has undergone changes since that post was written, and I don’t know if there are actually any extensions available there right now, so maybe disregard that part).
I believe the https://i7el.herokuapp.com/ website was intended to be more comprehensive, but it hasn’t been kept up to date.
I would check the Public Library if you are looking for the most “official” source of extensions currently available.