No.
They don't have to empty the bus and don't empty the bus.
It is
easier, safer and faster to load an ECV or wheelchair on a bus that is empty. So that is why they load them first at the parks and at individual resort bus stops.
We usually stay at OKW and are usually getting on at Miller's Road bus stop, which is the next to last stop.
By that point, the bus is seldom empty. There are almost always quite a few people on. Sometimes, the driver asks people to move from the seats that are needed to make a wheelchair or ECV spot on the bus.
Sometimes the driver asks people to please make room for us to get on.
Bus drivers do not require other guests to get off to make room for a party with a wheelchair or ECV. I have seen a few posts from time to time where people write that they were told to get off (one of the few was on this thread).
I think that those cases are misunderstandings of what the bus driver is asking. If the bus driver says things like,
"Can you please move out of these seats because I need to load a wheelchair?"
"Can you please step out of the way to make room for the ECV?"
Most people would understand those things as getting out of the way temporarily to make space or giving up those seats to make space. A very small number might interpret it to mean get off to make space.
Over more than 20 years of WDW visits with someone using a wheelchair on the buses, we have never seen anyone who thought that they needed to get off and not get back on when the driver asked them to move out of the way.
If people would need to get off the bus to make room OR if the bus is already standing room, our experience is that the driver tells us the bus is too full and we will need to wait for the next bus.
Some drivers do not want to load a wheelchair or ECV - we have occassionally been told by a driver that he can't ask the people sitting in the wheelchair securement spots to move (even when there are many other seats available). Even when the driver comes out and basically whispers to us, many people hear or know we need the spot and move anyway. We have seen more than one driver who told the people they don't need to move after they volunteered. 100% of those people move anyway.
Occassionally there is a group who refuses to move ( even when seats are available for them to move to). When that happens, we have to wait for the next bus.