Not so, Sunfish, hot air from the carb heater has less weight of air per unit volume, being warmer, and therefore there is less oxygen in this warm flow, so less fuel is burnt and therefore the rpm reduces. The mixture is richened by application of carb heat too, because more fuel remains unburnt in the oxygen-deprived flow.
But you and VP are correct in that carb heat does bypass the filter. I do not know of one installation where this is not the case.