what they were getting at is if your power goes out the pump in your sump will stop working. Meaning your display tank will drain until the water level is exactly even with your overflow that is drilled. When power shuts off on my 125 I have about 5 gallons of water that has to drain into the sump. This is because when power stops that continuous flow overflow does not stop draining when your pump stops.
If you don't believe what people are saying, try cutting power and see if your sump overflows, my guess is it will if its still that high. Dont do this unless you are prepared to clean up the mess it is likely to cause.