That might have been a credible argument if the github platform were open source (as is the case with gitlab), but in absence of that it's fair to compare to similar free offerings like sourceforge and bitbucket.

Providing free hosting didn't win sourceforge or bitbucket or any of the other firms a multibillion dollar payout. What did was capturing the mindshare of open source developers and hosting high-profile projects. Ultimately, the value is derived from the fact that "everyone is using it" and the ecosystem built around it (because everyone is using it).