At the face value, this statement is not true by any means. Existence of marine catfish family Ariidae alone disproves it. Many f/w cats visit estuaries regularly or even inhabit them. Finally, I have no such experience and neither have I heard a known, reputable expert to state this.
There is really no such thing as dosage - salt is used at wide range of concentrations. Surely, one needs to be careful with any fish, not just cats.