The mental model you present in your question is absolute, that salt hydrates or dehydrates. The truth is somewhere in between those extremes.

In this case, your body has a certain salt concentration it likes to maintain, so when you hydrate, it will try and maintain that concentration. If it can't it will either reject some of that water (if there's not enough salt) or require more water (if there's too much salt). A fluid that hydrates optimally contains the exact salt concentration your body prefers.