I think with a lot of these health trends, it doesn't take account for the fact that humans are animals.
Animals can't always guarantee they'll get enough water, or enough food, or the right kinds of food. Bodies aren't like a machine that requires specifically calibrated amounts of inputs. Instead if you drink too much you'll pee out more. If you drink too little your pee will be more concentrated so your body retains more liquid. If you get more nutrients than you need, the extra will be eliminated.
It's probably true that with many things (except calories) it's better to get a bit too much rather than too little. Maybe it's true that for water it's better to get nearly the maximum rather than slightly more than the midpoint amount. But, it seems unlikely to me.

