Actually, GitHub is just a supplemental CV. A potentially positive indicator, that among many others (including a CV itself) will be part of the body of work to determine a candidate's qualifications.
Those in hiring positions need to use various indicators of what would make someone a good hire. For most of history in the tech world, this simply meant a standard resume/CV, followed by a phone screen and then an interview where the candidate may be asked whatever questions are deemed relevant by the interviewer. Could be tests, exercises, etc - depends on who you were talking to.
Fast forward to today, where we have more ways that a candidate may show 'indicators' of talent. What about participation in user groups and meetups? Anyone can go to these things, but people who choose to go to them may have a bit more curiosity or interest (and admittedly free time) than those that don't, and that curiosity often goes hand in hand with talent. Not always, but again it is one indicator.
A healthy Stack Overflow reputation score might be another indicator. People who know nothing about programming probably won't be able to rake up major points there.
What if someone wrote a book about a technology? Another indicator probably, and if the book became a best-seller that would be even a stronger indicator since others are judging the material as worthy of their money.
As the author points out, all of these things take time, and many in the industry don't have that kind of free time. Understood.
Experience working at a known entity with a high barrier to entry is another indicator. We know that if someone passed the grueling interview process at certain firms, chances are they will get past our process as well. Another positive indicator.
There will also be some false positives. Candidates that belong to several meetups and have very active GitHubs may not be able to code.
Someone who has never heard of GitHub (or say Node.js or Mongo or whatever may be current and newsworthy at the time) will probably be given a negative indicator Would you consider hiring someone who had never heard of these things? Perhaps not.
Candidates without families that we might expect to have more free time may not choose to spend it at meetups and building GitHub repos either. I don't think we should immediately assume that they are less qualified than the ones that do, but I don't think any will assume they are more qualified.
The author says "you can't judge code without talking to its author". Perhaps you can, but I don't think anybody is necessarily suggesting that you should. No one is hiring candidates based on their GitHub activity alone without interviews, just like no one is hiring anyone based on their CV alone without interviews.
Can we just agree that all of these positive indicators are just indicators? As long as we don't use the absence of them as a negative indicator (as a measure of fairness to those who lack the time or desire), we are not doing anyone a disservice.