Not necessarily. The battery may be able to provide power to the grid when it's low and does not enough power to drive the bus. The bus probably requires some minimum voltage to move and a bit more to be able to climb hills. And of course you don't want to run out of power so more buffer for that. So it's not unimaginable a bus could be near empty for the purposes of driving the route but have useful juice to give the grid. Fair question would be how such discharging affects battery lifespan
