imagine a glass of fruit juice contained the equivalent of 5 tsps of sugar. If you drank it all, you'd have had the equivalent of 5 tsps of sugar. Imagine the same glass with half juice and half water. If you drink it all you'll have had the equivalent of 2.5 tsps of sugar
If you have half a litre and add water you nearly got a gallon. You wouldn't 've drinking that whole gallon would you. Just a glass full which contains little juice/sugar & water.
For example, without adding water you'd be drinking half a litre with a sugar concentration of, say, 10%. By adding the water it may reduce the sugar content to, say, 7%.
Won't said drink taste only 1/2 as good though? Maybe not half as good, but surely it won't taste as nice if it's watered down?
I suspect that Bazile was questioning it because most people know that if you water something down, there will be less of the original ingredients afterwards, not just the sugar.
I guess if the drink is quite strong to begin with, you could water it down a bit, even if you only added a tiny bit of water.
If on a monday you drink 2 litres of pure fruit juice but then on tuesday mix 1 litre of fruit juice with 1 litre of water you have halved your intake of sugars from that drink.