I am a native speaker (well, Scottish, so some may disagree ;), and I didn't get the title either. Maybe it's an American thing, by "office hours" to me means 9-5, local time - you know, the kind of hours that people typically work in an office.
> you know, the kind of hours that people typically work in an office.
It's exactly the same thing. There is no difference. This guy "works" in a home office on his project and he does so by talking with contributors (his customers essentially) instead of just sending a message on a mailing list or by posting on an issue tracker.
Where do you see the difference between this and a lawyer being open from 9-5 for customers? They are still his office hours and "office hours" primarily matter to outsiders who have to schedule an appointment.
For example, I work on a project that heavily depends on "library foo". The guy who wrote "library foo" (who is an employee of my company) holds "office hours" for one hour every Monday, where he commits to being in a Zoom call that other employees can join to discuss questions about "library foo". That does not mean he only works for one hour a week.