Indeed. I've been using github lately too, writing code for a charity even, but it's not code that can or should be open sourced.
Github-ism is a bit like credentialism, it lowers your risk but risk is where the interesting stuff happens.
There are two ways to hire, the lazy risk-minimizing way and the non-lazy investment minded way. If you put a maximum amount of effort into trying to reduce the risk of hiring you're going to cut your potential RoI off at the knees. You only hire people that are currently employed, because you want to rely on someone else's work in evaluating a potential employee. You only hire people with the right credentials who have relevant experience in exactly the technologies your company is using right now. You only hire people with very explicit demonstrations of their coding capability. Candidates that have such an easy time proving themselves are naturally going to be the easiest to hire. Not just by you, by everyone. Which means it's going to be more expensive to acquire them.
You should still acquire employees such as these, but if that's all you're doing you're throwing away a lot of potential. If you can manage to put together enough internal talent to be able to interview candidates even remotely well enough to make an independent and more or less objective check on their skills and capabilities then that gives you a potentially very significant competitive advantage.
Some of the best coders I've worked with came from diverse backgrounds and didn't have the "right" credentials or resumes. Several of the smartest and proficient programmers I've worked with have not had college degrees. Many others didn't have CS degrees.
Being able to find these folks and hire them because you can tell they have talent is a great opportunity to extract RoI from the hiring process, which is normally seen as just a cost-center.