Yeah, I'm just saying-- a big desalination plant is a solved problem. Grid scale energy storage has been ten years off for almost as long as nuclear fusion. The original author just hand waves their way through what would be the biggest energy breakthrough in a hundred years to save a couple bucks running a desalination plant.
(1) Ensure you always have fresh water available since you're continuously producing it.
(2) Reduce the size of desalination plant you must build by allowing you to run it 24x7.
You're right that you can just store the fresh water, so batteries aren't needed for #1. But the article is talking about how to achieve #2.