Luckily GB can explain things better than I can!
A potato isn't a special thing that selectively absorbs only salt. It absorbs salty liquid. So does a sponge. When you add a potato to salty liquid and take it out, you have removed a certain amount of salty liquid. But you could have done that much more simply by ladling out a bit of the liquid.
The concentration of salt will be basically the same, except if you keep the potato in whatever you are making. In that case, you have added additional unsalted mass to your food, which, in turn, dilutes it's saltiness slightly. But you can add any unsalted foodstuff and have that happen.
Potatoes don't address the dilemma of overseasoned food.
Here's what one of America's top food scientists has to say about it: What Einstein told his cook: kitchen ... - Google Books