The Chinese government is telling the world with this attack that, if you choose to interfere with Chinese sovereignty by means of the Internet, or to enable those who would do that, then there’s a cost.
Note the way the attack is targeted. It’s not just an indiscriminate DDoS of Github, although that’s been the effect — instead, they’re aiming specifically at two repos whose content, being designed and built with the aim of circumventing the technical means by which are implemented a significant goal of China's domestic policy, enables no more or less than a direct attack on the sovereignty of the Chinese government.
To use that content is to say: "You may not run your country in the fashion you choose, because it does not suit me that you should do so."
To host that content is to say: "In this matter, we take the side of those attacking the sovereignty of the Chinese government, by making it easy for them to share and improve the tools with which they do so."
What you're seeing, then, is the quite reasonable response of the Chinese government to these statements. Yes, it's annoying for those of us who use Github, and no doubt it's much worse than merely annoying for those who administer Github. That is the point. Github is being encouraged to consider how much it's worth to them to maintain the stance they've implicitly taken in this matter. I'm looking forward to seeing how they respond.