It looks like you're right about the history. According to the git repo, libgit's first commit was in October of 2008 while Github was incorporated in early 2008 (according to Wikipedia).

Github's popularity was probably due to Git's popularity in the Ruby community which may have been due to the official support of the physical model and simple protocols.

That said, an "officially supported physical model" is an API even if I originally had libgit in mind. Also, none of this invalidates the broader point, which is that Git won because of Github, not because of user experience.