You're right, arkana. Some people have used a deep sand bed to create the effect Telek is describing, but a canister filter will never be an anaerobic zone. Saying that there is no gas exchange in the canister itself is not at all the same as saying there is no oxygen in the filter. There is plenty of oxygen already in the tank, or your fish would be dead. Oxygen is constantly dissolving at the surface of the aquarium, and it's taken into the filter along with the water in which it is dissolved.
I suspect the oxygen argument is just a marketing gimmick on the part of the wet/dry manufacturers. What they're telling you isn't strictly untrue, but if fish can live in your water then so can the bacteria.
Most people just remove the nitrate through water changes since fish can tolerate a relatively high concentration of nitrate while ammonia and nitrite need to be pretty much zero. The anaerobic filter concept is also dangerous because if disturbed it will put some very poisonous chemicals into the water.