I spoke with Airbus about this and it basically comes down to:
The maximum brake temperature for takeoff is 300°C, as indicated in the FCOM. Crew are advised of the limit exceedance by the associated ECAM warning (triggered when the temperature exceeds 300°C).
The 150°C limit given in the FCTM is simply to ensure that any hydraulic fluid that may come into contact with the brakes will not be ignited in the wheelwell after gear retraction.
It does not ensure that brake energy limitation will be respected in case of rejected takeoff. That limit is addressed by the 300° warning.
So if brakes are at 250°C, you are authorized to takeoff. If you then have an an RTO, the brake energy limitation may be exceeded but certification takes into account the low probability of a rejected takeoff being energy max limited, with hot brakes and that there is some further margin available since energy max was demonstrated with worn brakes.