@jedberg is totally right! It's like judging people by the way how they pack their suitcase. To be perfectly honest, you can obtain metrics from Github that have nothing to do with coding, but behavioral analysis. I'll not go into details, because I think it's an unfair advantage.

Github is mostly used for personal projects. You can find a lot of dotfiles, frameworks, many smaller and some larger projects done for a hobby/friend/school/course or out of frustration/boredom.

What my Professor did is the way to go! Just hire that guy for a project, see how it's going and come together in a meeting to talk about the code, the solution and rationale behind the thinking. Then either conduct another team project to understand how he integrates in your company, or just hire that guy. Simple as that.