That’s not quite what a diuretic is. Diuretic makes you pee not poop. It’s also the case that people develop a tolerance to this effect. Caffeine does have an effect on your bowels as well, but that’s a separate issue to it being a diuretic. For example alcohol is a diuretic, but doesn’t make you poop more. MDMA is an anti diuretic that sometimes makes people need to poop.