I agree with you. They do indeed complement one another.
In fact, the multi-barrier approach was developed in the 1990s. The current drinking water regulations recommend, among other things, that there be less than one parasite per 100,000 litres of water. Now today, in 2023, this is impossible to demonstrate in a laboratory. A different approach was accordingly adopted, one that consists of analyzing the risks and then measuring the water quality. Water quality risks are assessed when it enters the plants and then barriers, by which I mean treatments, are introduced to reduce these risks.