The take-off distance on a paved surface is not influenced to a very great degree by water on the runway (not until you get to standing water of several cms deep). Landing distance is another matter of course as you are looking at decreased braking performance in that situation. The Note in the AFM covers the other situations where you do get an increase in rolling friction during take off: grass of various lengths for example, but does not cover the situations where a grass runway or an unpaved runway is wet. I suspect that Diamond did not test these situations and therefore will not be able to provide any factors for those situations. As for the why? Your answer is similar to what I would have given but I suspect that you could also state that Diamond did not consider the wet pavement situation worthy of a significant factor. Note that the caution only mentions wet grass or wet ground, not wet paved surfaces.
You could also try this question in the flight instructors forum or the private flying one.