And/or to build a local competitor. The more Github is down, the more it is considered "anti-China" in China, the easier to build a local competitor.
Or even globally. GitHub is, as you say, very important for the IT industry. Maybe China thinks it's too important. Make Github unusable for everyone else in the world and people, even outside China, might start looking elsewhere.
They can't shut it down fully, because doing so would shut down their software engineering capability (it has got that important). And people can host basically anything there, including anti-censorship software.
Attack feels like a power play to me. Either to pressurise Github into censoring for the PRC - "block the repos we hate, or we DOS your whole service again".
And/or to build a local competitor. The more Github is down, the more it is considered "anti-China" in China, the easier to build a local competitor.
This is all just my immediate hypothesis from the news. I'd love to see somebody who knows more about software development in China to say what might be really going on.