I don't think making that general statement is true. For some people they didn't solve the issue for good until they cleaned the coils. There was mention of a Toyota TSB in one thread and I believe the replacement part had a mold inhibiting coating on the coils which solved the problem for that car.
A wet filter does not automatically mean the smell, there has to be mold or mildew, and that certainly can grow on the coils (as well as other parts of the system).
Also the reason why when AC is on the smell goes away is not necessarily the filter drying out, but because our sense of smell works better in warmer humid air than dry cold air.
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