Of course a percentage of the sump will go through the uv if the uv is on a closed loop on the display. A percentage is better than all. Just a way to tip the odds in your favor, and make uv more effective. It's not a make or break thing, and uv isn't 100% no matter where or how it's installed.
It's like one loop from tank to sump, and another loop through uv, that'r connected at the tank. This same bypass looping is becoming popular again with hydronic heating. Did it to my home's heating system 3 years ago and my gas consumption went down 15%. Just simple physics making it more efficient.
