.docx is often very appropriate; it's one of the most widely used formats out there and is readable by pretty much any word processor out there.
Agreed that it's probably not the best choice for sending code, but the author's point wasn't that it's a terrible format, just that it's a terrible format for code reviews. PDF is equally awful for the reasons the author describes (lack of syntax coloring, inability to run the code, etc.)
Though IMO the author should just be explicit about what he wants; playing mind games with people is never a good idea, and it just self-selects for people who have used GitHub professionally. Choice of SCM system is almost never up to an individual developer anyway, are you going to ding someone with otherwise great credentials because their company uses Mercurial? It takes all of an hour to learn how to use GitHub anyway.
Anyway, I echo the comments I've seen elsewhere that a guy who got his first "adult job" at 30 then hired some people less than a year later is probably not the best person to take career advice from. This guy makes a lot of novice mistakes in his interviewing practices; so take this as a perspective on how some companies do hiring and not a definitive guide of the right way to do things.
