Im not saying your right or wrong about a skimmer working on freshwater but it would seem to me there arent as many organics in your average monster tank as there is in a pond. In an outdoor pond you cannot usually gravel vac so waste builds up more readily in the substrate. That is why plants can be so important to a ponds filteration, they use up the waste. Also the plants in the tank do decay and die. Leaves and other plant matter fall into the water and decay too. In a tank if you would simply remove the dead plant but this is not likely on a pond. These things make for more organic molecules in a pond than in a tank.
