It's possible that people with underlying health conditions (that might make them more sensitive to salt) naturally tend to consume less salt because of their health issues. Similarly, those with certain health issues might be advised by doctors to restrict their salt intake, leading to the "low salt = poor health" part of the U shape.
It's reverse causality for the low end. Almost nobody is on a low sodium diet.
OK, interesting to think about, but it seems a pretty wild hypothesis - not only do we have to assume these studies that revealed the U shape were so bad they failed to control for some massive health issues of some participants, the intervention itself of severely restricting salt intake failed to do what was advertised and still put these people on a rising slope of negative outcomes.
It's reverse causality for the low end. Almost nobody is on a low sodium diet.