Any company that makes most or all of its money online is the subject of DDoS attacks for blackmail purposes, github a bit more so because the Chinese government doesn't like it.
It's unfortunately a very normal thing these days.
What is the PRC's problem with Github?
They host the 'GreatFire' org and other software to bypass their censorship firewall.
Probably because Github is hosting code they don't like (e.g. code used to bypass censorship).