I think you're missing the distinction between "office hours" (what most people would read as 9-5 Monday through Friday) and "Office Hours", the term Simon is using to describe the 4-5 hours a week that he specifically allocates to having discussions with his users. Which is completely understandable given that there's not actually a consistent capitalisation difference.

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.

The article describes a practice for gathering feedback for your project: announce a set time when people can get on a call with you for a few minutes and discuss your project. The author calls this "office hours".

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.