No difference salt or chlorine. If it's going to rust it'll happen whichever is used.The magnet just means that the pool structure is steel. Mine is plastic so I really don’t know how prone to serious rusting a steel walled pool actually is. I think it’s not uncommon for some minor surface rust, but I have heard of people who have salt pools and steel walls having serious rust issues. When they pull that liner off you will know for sure. Your steel coping rusted away at the joints. Most likely it was galvanized steel which is basically a protective zinc coating that the steel has been dipped in to stop rust. The installer probably cut those pieces which would have taken off the zinc coating at the cuts which would have left those steel coping pieces vulnerable to rust. Hopefully your pool structure is fine.
Edit: I would just make sure that the contractor spray a bit of metal primer spray paint on the freshly cut metal before filling and painting the coping. This will help prevent it from rusting again.