I totally disagree , what you quoted from FCTM has nothing to do with power on brakes but more of having engines stabilized at 50 % of N1 before applying Flex or toga
Power on brakes will not save you from directional problem if both engines are not stabilized at 50% of N1 . This can happen with two different mounted engines , called intermix engines. Spool up time is different and you have to wait for both engines stabilized at 50% being on brakes or not.
If brakes can save you from a directional control ? Why rolling take off is allowed ? you apply immediately 50% of N1 , if you fear a directional control problem , that could happen to an aircraft spooling up not on brakes , I would be more concerned for an aircraft already rolling at lets say 10 kts , with an assymetry.
Having an N1 assymetry with an aircraft already rolling would be more difficult than an aicraft in standstill.
As an analogy for the 180 on the runway SOPs say: "On ENG 1, set N1 to between 30 % and 35 % (CFM engines).."
Here talking about actual N1
When Airbus say Thrust , they mean Thrust . When they say Thrust LEVERS it means Thrust LEVERS.