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.
Coffee is a diuretic so yeah if you over imbibe youre going to poop even if it’s the purest black.
That isn’t to discount your point that the stress lactose puts on guts isn’t helping.
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.