In case of the assumed temperature method (Boeing) or Flex method (Airbus) I agree that full TO would be available on moving the thrust lever to the TOGA detent.
However, the derating concept is something different since you are derating the engine for that takeoff.
There is some info on
http://www.b737.org.uk/assumedtemp.htm which could maybe explain the concept better than my posts. Having said that, I cannot comment on the B777 but I would have assumed that the concept is the same.