The aircraft requires less thrust (than is currently available) in the sense that at the higher temp. the engines produce less thrust BUT the a/c can still meet the performance requirements.
So, as temperature rises, there will come a point where the thrust available matches the thrust required to meet performance requirements. If you set exactly the same thrust in a lower temperature environment you will still meet the performance requirements. One way of doing this is by telling the system that the OAT matches this higher temperature limit. This is the assumed temperature.
The effect is that the engine produces an amount of thrust equivalent to what it would produce IF the temperature was at this higher amount however the thrust produced is still sufficient to meet performance requirements.