This doesn't actually seem to be completely true. The diuretic effect isn't enough to really impact the benefits if drinking a 99% water solution.

Although caffeine is a diuretic, coffee is net hydrating.

https://health.clevelandclinic.org/coffee-dehydration

Nope, only true in extreme cases. I was curious about that years ago, and while it might make you need to urinate more frequently, it doesn't actually make you urinate more volume unless you consume way more caffeine than your body is used to. Now, alcohol on the other hand, anything over like 3-5% ABV does have a net dehydrating effect.

Not enough to offset the amount of water you take, look it up if you want. It does make you empty your bladder and that might be more water than what you just drank, but then your bladder takes a while to fill again. It doesn't really make you generate enough urine to offset what you drank at all.