What makes you think Github is "large and well connected"? Don't get me wrong, I love GitHub, but due to their architecture decisions, they aren't anywhere near a great example of best practices in the industry for preventing DDoS attacks.
One that would really help with DDoS attacks is if they didn't do everything on github.com. Layer 3 DDoS mitigation will always be cheaper and more effective than Layer 7. If it's not obvious what I mean, here's an example:
