It's not. Unless you have hypertension or some other medical condition around salt, there's no reason to restrict your salt intake.
For the average person, the level of salt in restaurant food has no unhealthy effects whatsoever. Amounts in excess of your daily requirement are simply excreted in urine.
It does not, and that can (and has been!) be empirically shown. Salt gets into the food and changes both chemical properties and how it responds to cooking. Pre-seasoned meats, for example, are more tender.
If you prefer to salt at the table, by all means, eat your food however you like it, but it is simply false to claim that there's no difference or "chefs just add too much salt".
Many athletes especially in very hot climates need high sodium, sometimes up to 8 grams a day.
The reason a chef "knows this" is because if you judged their food with or without salt, the salt one would win, so in the end they're required to add waaay to much salt, when the tongue just needs those surface crystals.
Healthier to salt after, and therefore cook your own food.