Potatoes absorb liquid. So do sponges. They don’t magically extract salt.
Wolke on the subject: https://www.washingtonpost.com/archive/lifestyle/food/2001/04/18/can-you-save-a-salty-soup/72da6abd-defa-406a-8370-7e66f03719f4/?noredirect=on&utm_term=.b1c4398b05ec
"… Did the potatoes really reduce the concentrations of salt? … No. All it means is that the potatoes soaked up some salt water; they didn’t selectively extract salt out of the water. Would you be surprised if a sponge placed in salt water came out tasting salty? Of course not. But the concentration of salt in the water – the amount of salt per quart – would not be affected. So the salty taste of the potatoes proved nothing, except that for more flavor we should always boil our potatoes, and our pasta for that matter, in salted water, rather than in plain water.
Okay, now, what were the results of the conductivity measurements?
Are you ready? There was no detectable difference whatsoever in the salt water before and after being simmered with potato. That is, the potato did not lower the concentration of salt at all, either in the one-teaspoon-per-quart sample or in the one-tablespoon-per quart sample. The potato treatment just doesn’t work. Period."