To me (non-native, Spain), I read "office hours" as the time when I was a teacher in university: you have a set period of time where you are marked as available in your office for student consultation. I'm pretty sure that's what "office hours" is supposed to mean, although in Spanish and Catalan the word used would literally translate as "consultation hours" (horas/hores de consulta).

The phrase is used in the same sense at universities in England.

The term "office hours" in American English can mean either of these concepts, but in the current context it clearly means the one you suggest ("horas de consulta").

This is the same in the US based on my experience

In American English it can mean that too. The meaning of "certain hours where you work" and "certain hours where you have an 'open office'" overlap enough to be confusing, yes.

When you run your own organization you can set your own office hours.

> 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.

No, this is actually not what it means. "Office hours" has two distinct meanings: one is the hours you are in the office. That is not what is meant here. The other meaning, which is what is meant here, is specifically designated times when experts make themselves available for random questions about the thing they're an expert on.

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.