Well the built in boxes do not need the siphon maintained. When the pump is off, the tank drains to below the overflow. The boxes then drain into the sump, which is why you need that extra sump capacity

When the pump is turned back on, the extra water from the sump is pushed back to the tank and then spills over the top of the overflows again and the cycle is restarted.

I had an old siphon type and cannot remember that this was an issue and I am sure I lost power at times, certainly lost the siphon enough at cleaning time.