Pretty much al water in te Netherlands eventually flows out to sea. The problem the country is experiencing is that during periods of heavy rainfall the rivers are already at capacity because of water that comes from upstream so they can't pump all excess rainwater from the polders into the rivers. If you can temporarily store all that rainwater in the polder (by using wadi's or infiltration systems and letting it slowly dissipate into the groundwater) instead of immediately funneling it into sewers and into the rivers you can lower the peak excess by spreading it out over a longer duration into moments where the rivers do have the capacity.

Tldr, it still goes to the same place, just at a later date by temporarily storing the excess water.