The issue is that caffeine is a mild diuretic and depending on your caffeine sensitivity and other factors, that may have a more outsized effect on you in terms of fluid loss than the water you consumed with it.

So if you drink a dilute coffee (ex: weak iced milk base coffee drinks), you are probably fine but if you are drinking plain, hot coffee or low milk espresso drinks and you are sensitive to caffeine's diuretic properties, you are probably going to lose more than you gain.

> you are probably going to lose more than you gain.

There isn’t any science to support that. Feel free to only drink hot coffee for 3 days and see if you die of dehydration… Maybe you even think it's 2 days since hot coffee is even worse for hydration than nothing, apparently...