That assumes they put code on github. And that they publish the code. And that you know anything about their code.
Then there are those of us who use bitbucket. There's lots of code which doesn't get published because it's not relevant and if it is, it might be a one off project that I'd rather not make public rather than make it look like it's abandoned. Finally, some code just makes no sense out of context - some of the code I'm most proud of were just binary patches to firmware of an old mobile phone with delphi gui for hooking it up to specified functions. Without context that's just an ugly delphi form that does a weird version of "patch" operating on a text file and example files. In context, it's a lot more. Not only I wouldn't expect anyone to understand what that project was doing and why it mattered, it would not get uploaded to github because it was outdated before github even existed.
So while the quoted sentence is still true, I think anyone using this service misses out on masses of experienced developers who simply didn't publish their code on GH, for various reasons. If you want to find a standard <recent language> developer doing lots of opensource work, it might be a good offer though.
The difference might not be clear to the people paying for this service.