First of all, an SWG can't remain algae free with 3 ppm FC with 80 ppm CYA unless the pool is poor in algae nutrients. We've had plenty of examples of a 3 ppm FC target that simply doesn't work -- it's too close to the edge with high chlorine demand even if algae isn't yet visible. 4 ppm FC is pretty much the minimum required with 80 ppm CYA and as a minimum that's a number to be measured in the morning after any overnight loss.
Also, remember that the SWG is maintaining the FC level whenever the pump is running so if things are set up correctly where that is mostly during the day when FC loss is greatest, then the FC level is fairly constant. A manually dosed pool will fluctuate in FC up and down so targeting an "average" 4 ppm FC with 80 ppm CYA may have the FC get below that over long enough periods of time that algae can grow. In theory, an automated dosing system, such as a peristaltic pump, maintaining 4 ppm FC with 80 ppm CYA might work, but we don't have enough experience with those to know for sure.
Also, there is some superchlorination in the SWG itself that might help kill off floating algae though obviously would do nothing against any algae on pool surfaces. The superchlorination in the SWG is different than that from manual dosing using hypochlorite chlorine because the chlorine generated in the SWG at the anode is in very acidic conditions, at least until it meets the water that is very basic/alkaline from the cathode.