The engine doesn't know the difference.
Derate vs Assumed temp...
Derate can be used in conjuction with assumed temp to get even further reductions than would be available to you through assumed temp for takeoff, and then you get further temp reductions and protections throughout the rest of the envelope for flight.
Effectively an aircraft with Derated engines on them has engines that are too large for the aircraft. So its an efficiency loss. HOWEVER, that efficiency loss provides for a much larger thermal margin again extending the life of the engine on the wing and saving maintenance dollars.
Cheers
Wino