In exchange for that I get extremely high bandwidth feedback from real users of my software!
Per the article, he's not saying you should work on the project during office hours. Instead, to get feedback from users, he's allocating some time where users can have a talk with him about the project. He's calling that 'Office Hours'.
I think that it's a really valuable suggestion.
Of course the latter is still time spent working on the project, but it's certainly not in direct conflict with having a full-time job paying the bills.
It's a useful piece of advice for people who are in need of such feedback, and can afford to block off a bit of time on their schedule on a regular basis. It doesn't apply to everyone, but it's a good trick if it does apply to you.
Are you certain? I'd think most open source development is done by paid developers, on company time.
Assuming that everybody has the luxury to have additional office hours available is a bit far from reality in my opinion.
I mean, if you can offer office hours for an open source project you probably are already so popular that you are able to work on it fulltime, right? And if you're not that popular, you cannot offer office hours due to your daytime job; as you would have to decrease the rest of your remaining free time that you probably need to sleep and eat.