"Sensors inside the damaged left engine detect that its spinning slower than normal which triggers a reduction in fuel flow"
This means that as the a/t is d/c the fan blades spin slower than they would be at the position the throttle is left at due to the debris. Therefore the a/c has sensed the loss of power in the spinning blade through its sensors and in response reduced the fuel flow to that engine. In brief, the a/t was forcing the blades to spin to create power for that engine, whereas when the a/t is d/c although the throttle is still where it was, the a/c has sensed a slower rate of spin and automatically decreased the fuel flow. Basically the a/c reacted as if the pilot commanded a decrease of power in the left engine through a series of automatic sensors, when he had not and shut down the incorrect engine.
Hope this helps.
FS