In fairness even many years ago we found due the the colder Winters and more salt on the roads there was more rust in cars from "up North", the only comparison down here is as you say salty coast lines. We had a customer who owned a beach shop by the pier and used to park his car there all the time, he could ruin a car in two years.What a load of nonsense, like you don't have a salty coastline in England for Wales.
Another one was Motorbility ones where the old dears used to park on the sea front in the Winter with the spray blasting over them!
I have noticed the extra rust on brakes and suspension etc. on photos on Forum from North of the Border in comparison with similar age vehicles down our way. Even at auction cars seem to fetch higher prices down here, although I have yet to buy a decent vehicle from Cornwall, it is as though they get every inch out of them before selling, a bit like farmers.