Hi,
this is correct - the relais holding the connection to the generators are held close by battery bus current. Once the battery drains, you will loose all power to the aircraft. This will usually not happen, because the battery charger is powered through the ground service bus.
If you turn off the battery switch, all buses will become unpowered, except for the hot battery bus.
If you want to fly without AC generators you can - the battery will power all the buses you mentioned (but while on the ground, you need to move the standby pwer switch to "stby pwer", not to auto). But only if the battery switch is on.
Once the battery is drained, you can not connect any power to the aircraft, anymore (not even GPU).
Jan