@Lancman
There a a lot of misinformation sculling around on this.
The final report states that the overfill was 300 litres (237kg) put into the engine feeder tanks 1 to 4. These tanks are grouped to have approximately equal moment about the CG so if it was, as seems likely, 75 kg in each there would have been negligible effect on the CG.
There was no overfill into tank 5.
It is all a long time ago, but as SSDriver says the overfill capability was probably there to cater for extended taxi or waiting time operations.
In this particular case the dispatcher ordered 2000kg rather than the standard AF allowance of 1000 kg to be loaded for taxiing presumably because he/she believed a more distant runway would be used because of maintenance work, but in the event the pilot asked for and was given the usual runway which meant that the aircraft was overweight for take off because only 1000 kg of taxi fuel was used. [Plus of course the additional baggage]