Woods Hole is a fine research institution. I think you can assume they were trying to make the concept accessible to layman. And I think they are trying to not

Woods Hole is a fine research institution. I think you can assume they were trying to make the concept accessible to layman. And I think they are trying to not get into all the complexities (yes, Na+ and Cl- ions are added/removed in some natural processes, but many times less readily than water). One could easily point tectonic processes and salt loss via evaporite layer deposition, but you don't lose a lot by not going into the weeds.