I'd say that evaluating someone's GitHub account is way more meaningless than evaluating their certifications. Not everybody open sources all their stuff, nor necessarily uses GitHub. And most of the work one does, is usually for a company and can't be shared anyway.
What you say is very true. Furthermore, there's no guarantee that code associated with somebody's GitHub account was actually written by that individual. It's useless to gauge the ability of a developer based on code in his or her GitHub account that somebody else wrote.
