That's bullshit...if that were the case, then everyone who adheres the use of a thermo stack that includes caffeine in it would not be getting enough water. This is simple...When diuretics are included into the diet, one must expect an increase in water fluid loss from sweating, pissing, etc, but this does not mean the water consumed with it is not counted. You could take a caffeine pill and 16 oz of water, and it would still count towards the daily consumption for water. The same applies with tea. You can count the water in all foods (milk, tea, etc) to meet your daily requirements, but for most, they just exclude this intake to be on the safe side. Most will not count it towards their daily intake even though they can. However, most will drink an addition gallon or 2 outside of this intake to be on the safe side. You can almost never get too much water so the added amount doesn't hurt.
BMJ