"How is it possible to overdose on something, when any surplus of that something is ELIMINATED through the urine?"
Have you ever tied one on and consumed enough alcoholic beverage to cause you to vomit? Sure, drinking causes copious urination, but the damage from alcohol poisoning is on a CELLULAR LEVEL. It takes some time for the body to process anything, even water-soluable substances, and discard excesses.
Please, don't fool yourself into thinking that anything and everything is water-soluable and easily eliminated via the urinary tract. One can overdose on Vitamin A and cause permanent visual damage - yes, Vitamin A.