Yeah I think the intention is to prevent someone who disembarked from the bus and are then crossing the street from getting mowed down by a car overtaking from the bus's left. It's similar to why you can't pass a school bus when it has its stop sign out.
It was clearly a typo in the book. There’s even a diagram showing that people are loading from the right, and a sentence saying you pass on that side while passengers are loading.
If they meant “don’t pass while it is stopped”, they would have said that instead of writing the equivalent of “you can pass when [false]”
