Your friend is an exception. I've met a few, too, but the majority of top-tier programmers I know program outside of work. I'm not talking about "hackathons" and shit like that, or github profiles--usually just personal projects for practical or amusement (often both) purposes.

Heuristics are, unfortunately, quite valuable in hiring scenarios, especially for start-ups.

It's also just much more fun to work with people who love programming, and an easy way to tell (even if it's susceptible to a few false negatives like your friend) is to find out if they program in their own time.