That is correct Empty, salt helps against Nitrite poisoning.
Mollies are a debatable subject. It is true that they have been bred for so long in freshwater environments that they do not need salt. But if they are wild caught, they require marine salt, not freshwater salt. Most, if not all, that you will come across are tank bred.
There are two types of basic salts:
Freshwater: Used to fight off ich, nitrite poisoning, and other sick illnesses
Marine: Used for brackish and saltwater tanks.