The research was seriously misreported--you don't actually need to drink anything like that much, the number is for total dietary intake in any form. With enough juicy food there's no need to actually drink anything. I'm reminded of one day my wife saying she was thirsty, please cut a slice of watermelon. At first I thought it was an English mistake (it's not her native language) but that's really what she meant.

I don't know how true it is and I don't remember the source, but I recall hearing somewhere that you also get a lot of hydration through food sources. So it's not necessarily that you need to drink a lot of water to stay hydrated.

I guess that might also be why people say to drink more water when you have diarrhea. The absorption of water that usually happens during digestion is not occurring.