The number and conclusions are inaccurate.  

What is coffee?  What is tea?  What is water?  What is soda?  What is diet soda?

Coffee is water with a ground bean in it.  If you swallowed a couple of those beans and drank a 12oz glass of water, does it count towards your water intake?  A cup of coffee is generally under 100mg of caffeine.  That means even if the number involving liquid loss was correct, you would still hydrating at nearly 1 cup for ever 1.5 cups you drank.  

Tea is water with an herb in it.  If you ate the tea leaves and then drank the water, would you count the water?  There is 26mg of caffeine per cup.  This means even if the incorrect caffeine dehydration numbers are to be believed, it is like drinking water.

If you eat a candy bar with a water, does it negate the hydration?  No, that is stupid.  Neither does a table spoon of sugar and a small amount of caffeine.  Diet sprite, ginger ale, and the like will hydrate you exactly like water will.  Diet caffeinated sodas are nearly identical too.

Finally, the loss of water doesn't occur at that rate for people who regularly drink caffeine.  In all likelihood, if you wake up today and drink 4 cups of coffee then you probably do that every day.  The loss is minor to you because your body processes caffeine differently.  

If you put a packet of flavor in your bottled water then you have just made diet caffeine free soda.  Carbonation doesn't change hydration.  

Just some food for thought.