Diesels have idle governors that automatically cause the injector pump to deliver more fuel if the engine slows down. You don’t need to give it the gas, the engine does it automatically in response to the additional load.
Governed gasoline engines act that way also, (farm tractors etc.) The “throttle” on a farm tractor does not directly open the throttle, it sets the speed the governor holds the engine at. You let out the clutch and the engine automatically opens its throttle in response, you disengage the clutch and the engine automatically closes the throttle to prevent the engine from running away.
Your driving instructor was correct.