I always thought epsom was a salt too, and objected to adding it to plants until I understood more about what it does. It's still not a good idea to put a lot of it in your soil, because too much Mg can cause severe lockouts of other elements, but rest assured it is not salt.
Here from the web:
It's a chemical compound made up of magnesium, sulfur, and oxygen. It gets its name from the town of Epsom in Surrey, England, where it was originally discovered. Despite its name, Epsom salt is a completely different compound than table salt. It was most likely termed “salt” because of its chemical structure.