I also agree with some of the other comments - Github certainly isn't the end all be all. But, we're all trending away from traditional education, and software developers are on the leading edge of this trend. There may be a fairly standard path for "finance guy," but great developers can (and often do) come from any background. Resourceful developers can learn all they need for free via sources outside of a CS degree someplace.
So the question for a company seeking great developers becomes "how do I evaluate these non-traditional folks?" Resumes don't get there, but open source work can work to fill that gap. It's certainly not key for everyone - there are plenty of folks who stand on their own without a public repo in site. But, increasingly, it replaces more traditional indicators of talent like CS degrees.
Just my 2 cents, and I'm admittedly a bit biased.