I see it as one of those "strong signal if it's there" things. If someone has loads of stuff to show you and they clearly love programming because they're doing it all the time, I feel like they are gonna be a great programmer. But of course if you're not in a position to show stuff, that doesn't mean you're bad.

My guess is most people are covered by some sort of IP clause for their main work, and the most visible OSS projects are still not run by very many people, so most devs are not gonna be able to show you much.