Nothing is "obvious".
For all we know this could be a bored teenager sitting in his basement in New York, Paris, Tokyo or Moscow.
It could also be a false flag operation by the NSA, to massage the western adolescent nerd's mindset into quietly accepting their next round of mass surveillance ("Must protect from the evil chinese!").
The Chinese openly attacking GitHub is the least plausible scenario, unless you think they are stupid.
At least as guilty, imho.
And who is "they"? Baidu? The Chinese Government? The great firewall administrator himself?
Sorry but how would a bored teenager somewhere have control over injecting attacks in traffic passing through the "Great Firewall of China". This really doesn't seem like a typical attack to me but you are right in that I should not be claiming this an obvious attack by a nation-state without the facts.
The same way a 15yr old once took out Yahoo, CNN, eBay, Dell and Amazon: Dedication.
Perhaps it's even a Chinese activist, looking to draw attention to the firewall or to spark a little diplomatic quarrel with the US?