The following is my understanding, im not in the field.
The plane was never overbooked. Instead, the United transferring crew showed up at the gate after the sold out flight was already seated.
Transferring employees trying to get on a sold out flight is not considered an overbooking. In this situation, passengers can choose to give up a seat, but they are not required to.
Passengers can be removed from a plane for safety reasons and such, but overbooking must be resolved before the plane is boarded.
