I think its about release which helps speed. Light wind boards have really wide tails for planning power but that leaves you with a long trailing edge which has more drag . The cut outs help reduce the effective area and speeds things up. It must work because it has been around for a long time.
Nearly every windsurf Formula race board has the cut outs, so they have been established for many years and therefore must work on a windsurfer. The slightly wider release edge length while still having the fin back and getting more control from the narrow tail seem like things that should help a kite raceboard too. The cut-outs are used on many performance slalom and freeride windsurf boards too and are difficult and expensive to include, so they must work for those boards too.
Johnny Heineken had a Mike's Lab board with cut-outs which could be filled with blanks but seemed to prefer the cut-out blanked. I'm sure if kiteboarding had gone into the Olympics there would be no doubt whether cut-outs worked or not as they spend fortunes testing much smaller details in Olympic disciplines. I would expect the cut-out to affect the trim of the board and the positioning of the footstraps and possibly the fins, so I'm surprised that they didn't have two separate boards to compare. You can see Erika is using it with the blanks in at 6 minutes in this video.