My guess: he just positioned the foil parallel to the waterline - but this is not zero angle of incidence because the flow is tilted upwards. If you want zero incidence and the rudder is shallow, place the foil parallel to the nearby hull bottom. If the rudder is VERY deep, horizontal is fine. If it is average, place the foils between horizontal and parallel to hull bottom.

Related info: my rudder foils are positioned with the flat face parallel to the waterline. This represents about 1 or 2 deg incidence,if we consider "zero" to be the zero lift flow direction. However, the flow there runs upwards. Assuming the flow being paralel to the hull's bottom, I must add another 11 to 13 degrees, totalling 12 to 15 degrees incidence, which seems VERY HIGH.

The lift must be significant. The price is paid in drag. I wonder if this incidence isn't too much - and plan to try a plain blade as well, just to compare performances.