The term boarding has broad interpretation. For airlines it is generally applied to the whole process of loading passengers on the plane prior to takeoff. It is not the limit line of actually stepping on the plane and yelling "safe!". Even if you are on the plane, sitting and buckled, until they close the door everyone is still boarding. I wouldn't be surprised if it extended until wheels up such that a return to the gate to eject a passenger would still be considered boarding denial.