On the other hand, I don't think it's really necessary to prove with technology that the 中國人民解放軍 is behind this. Diplomatic logic is sufficient. The behavior is simply an internet equivalent to jamming the Voice of America.[1]
Github is broadcasting. The 中华人民共和国 has a sovereign's policies regarding broadcasting. The 中國人民解放軍 executes those policies. Github operates with a business model that ignores sovereigns at its own peril. Calling one sovereign for aid when dealing with another sovereign also carries peril.
Allowing political content in an online community always comes with the risk of trolling and flamewars. A hands off editorial policy only means Github hasn't made a tough decision about what the Github community is not. Decision day can only be put off so long.