Applecrumble.
The FCOM is saying it does not recommend speed brake to further reduce the speed as you will be slow speed (reducing to F Speed ) at Flap2 anyway and the airbrake is fairly ineffective for speed reduction.
Therefore dropping the gear will be far more effective in reducing energy.
The following statement ref slight oscillations is just a statement of a known Airbus software bug NOT a reason for not taking speedbrake with flap 2.