Multiple other reports described it as trainee, trainer, and two passengers. (Technically, the trainer was a passenger).
I suspect it's translation issues. Some reports were the bus "fell" into the river, as though it happened on its own and the driver didn't cause it.
Expect more situations like this. It turns out the Unbound Prometheus is in fact very bound by the decreasing number of people who are willing and able to operate such items and processes.