I don’t follow your point. GitHub is a private company that provides gratis services to many open data providers. The underlying git repository is non-proprietary and highly portable. The value-added features that GitHub provides (like issues and pull request review) are exportable and migratable to other systems. In short, there is very little lock in. How then is government open data subject to the “licensing models” of GitHub? GitHub claims no ownership over the content it hosts.
Don’t get me wrong. I would prefer an entirely open source & distributed solution to data & source code hosting. There is a large reliability risk of centralized hosting. But in no way does GitHub’s prominence undermine the promise of openly licensed data.