There is an op amp in switching power supply
It is an error amplifier. The amplifier provides an analog feedback signal through the LED/phototransistor to control the PWM across the isolation barrier. 

Here is what a real circuit typically looks like (schematic from this TI document): 



Since the circuit is powered by the isolated voltage, the LED is initially 'off' and the PWM runs at maximum output. As the voltage approaches the target voltage the LED current increases, causing the PWM to back off and (if tuned correctly) keeps the voltage from overshooting the target, whilst minimizing the rise time. I agree that your block diagram appears to be reversed from what is normally possible.  

The three-pin TL431 contains both an op-amp-like amplifier and a fairly accurate and stable band-gap reference, all for a penny or two!