twistedenginestarter,
the engine itself does not produce all the thrust. Try to picture the whole assembly without the turbomachinery. What you get is a ramjet which has only a diffuser, a combustion chamber and a nozzle. And, as we all know a ramjet does produce thrust at high speed.
Even better, Philip Hill and Carl Peterson write in their book 'Mechanics and Thermodynamics of Propulsion': At a flight Mach number of 3.5-4, the thermodynamic analysis indicates that the compressor is no longer needed; the most efficient engine is the ramjet.
Isnīt that what happens with the SR-71? Donīt the engines change to ramjets at high speed?
I reckon, you confuse thrust with energy which is put into the engine in form of jet fuel. Think about it, without a supersonic diffuser and converging/diverging nozzle there is no way you would go supersonic in the first place.