Yes, the idea behind the office hours is that you can point people at your calendar. If the calendar is full then that's how it is. Maybe the team has to grow to accommodate the increased demand but the maintainer certainly won't spend an extra hour working on the project beyond the time he allocated.

Compare that to the usual issue grind. You feel compelled to respond as fast as quickly and maybe even spend your whole day because you know that the backlog will grow worse tomorrow.