.. and the thrust so described is due to the application of gas pressures to the abutting structure.
The graphic gives the story for one engine ie internal story.
Now one has to look at what is happening in the intake system forward of the engine inlet face and the exhaust ducting aft of the turbine.
With a supersonic aeroplane (particularly) there is a tremendous amount of energy in the incoming airflow. As Owain observed earlier, the engine can't accept this supersonic airflow. We have a quite critically designed and controlled set of gadgets to provide a tightly managed system of oblique (ie the shock wave is inclined rather than normal to the direction of flow - such shocks are better for energy losses) shocks (shockwaves) ending up with the flow going through a final relatively weak normal (perpendicular) shock wave before it is massaged onto the forward face of the engine compressor inlet.
The engine itself is a comparatively highly strung thoroughbred stallion and doesn't take kindly to operating conditions which are too far off the design operating conditions case .. hence the various means of dumping excess air to keep the horse happy. With a bit of careful design, the varying air pressure can be managed to create a nett forward thrust on the whole of the inlet and outlet system.
Hence the figures which have been quoted indicating that, at high supersonic speeds (especially), the engine contributes only a small proportion of the total thrust pushing the aeroplane along ...