I think the salt is actually worse for keeping thing cool longer.
Most of the cooling capacity of the ice is in phase change from solid to liquid. The salt is moving it to -10 which means bigger gap from outside temperature. So the cold escapes quicker.
If you use normal water it will climb to 0 faster, but stay there longer.
Salt for ice cream makes sense because you need the lower temperature.
Question is if it lasts longer. Couldn't find any info on that.