That is not true. Salt.added to ice reduces the temperature. Measure it. With the right mixture you hit the eutectic temperature of the mix which is -21 C.Ummm, adding salt to ice doesn’t do anything to the temperature. It lowers the freezing point, which allows it to be liquid at the same temperature that it had been frozen before salting. A salty slush can be 20° and liquid. Being liquid, it transfers heat away from the tuna much faster, leading to colder fish. Keep in mind that it also leads to a faster transfer of heat from the sides if he fish box.
Add enough salt and you can freeze the tuna!
Google "temperature of salt ice mixture"