Open-collector current measurement
The circuit you refer to Data Sheet  (page 23, is merely an output signal to a user that an error has been detected.  The detection circuitry is another matter.  

The manufacturer has spent a lot of time and money in developing their detection circuit.   

"How would such circuit be implemented to be inexpensive and reliable?"  

Achieving Reliability is a broad subject involving both mechanical and electrical design.  

The manufacturer states, "The error output of SKiiP®
3 V3 is short circuit proof.".  Using an open collector means you can short the open collector to Ground with no ill effects.