I disagree. Github arrived in 2008, soon after many DVCS options arrived. Mercurial and git were developed at very near the same time in 2005, Bazaar was developed around then too. I don't believe that git had anywhere near the market share back in 2008 that it enjoys now.
When github arrived, git's biggest win was the kernel. But that was kinda a given if you consider its genesis. Granted, it's a noteworthy project that helps prove git's scaling capability. But IMO git didn't win until years later when folks who had adopted the other DVCS engines started switching to git.
Don't know about the rest, but I switched from SVN to git before I even heard of Github for my personal project and also in the company I was CTO at the time.
Two things were decisive: being able to commit and browse the whole history while offline and the fact that my main dev machine was Linux so it worked faster than anything else.
When github arrived, git's biggest win was the kernel. But that was kinda a given if you consider its genesis. Granted, it's a noteworthy project that helps prove git's scaling capability. But IMO git didn't win until years later when folks who had adopted the other DVCS engines started switching to git.