It's not saying water does nothing to get water back in the body; it's saying that dehydration is often caused by a physical problem or something of the like that water may not help. And that bottled water isn't the only way to rehydrate, do making the claim in an effort to make it seem super special because of that isn't exactly completely right... Or something.
I don't know, maybe I'm misunderstanding it, but I don't think it's saying that water doesn't rehydrate. If I'm wrong, I'll happily admit it. Wouldn't be the first time. <3
On the topic of the article, what's this about bent bananas and curved cucumbers?