That makes no sense to me. If we follow the logic that any mixing of salt with fresh water has a negative effect on water balance then there is no natural source of potable water on the planet Earth. All natural water conatins some salt, yet people manage to regulate their fluid levels perfectly well by dirnking it.
Quite obviously there is some level at which drinking dilute saline has a positive effect on water balance. The question is simply one of what level that dilution has to be.
It seems like the article quoted has decided to play it ultra-safe and simply declare that even diluting sea water to 1ppm with fresh will not increase the total amount of physiologically available water.