Increasing flow and decreasing concentration through most filtration assemblies generally increase their effectiveness. There are some exceptions in certain active transport systems that I can think of and the kidneys certainly do some of that, but even then the system has to be operating above a certain optimal flow.
Healthy skepticism that this makes a difference in day to day human health may be warranted, but skepticism that the claim is even meaningful in the first place is not. Your body does routinely contain a lot of toxins, and it has mechanisms to clean them out. “Polluted” is not a binary yes/no attribute anywhere else in the world and it hits not binary in your body either; it gets more or less polluted as it excretes less or more waste.
Sure, if you’re dehydrated, your liver and kidneys will probably have a hard time doing their job. But drinking more water than you need, drinking magic teas or vitamins, etc does nothing more than what your liver and kidney normally do in a healthy body.
Metabolic byproducts are literally toxic, and your liver and kidneys remove them constantly. "Detox" is probably overused but your comment is wrong too.
