Couldn't disagree more. They're a successful startup, but definitely still a startup.
What else would they need to do in order to be pass the start-up mark?
Github will remain a startup until they can make a dent in the big boys' bottom line, for example: Perforce.
Github is insanely successful amongst startups and hackers, but has yet to significantly penetrate the enterprise. Hopefully this will change (for us and for them) because their product is a gazillion times better than anything else out there.