I don't buy that at all. Clams are filter feeders and can remove detritus and free-floating algae from the water column, but they still digest food and produce waste which will end up as nitrate, just like any other creature in your aquarium. Apart from that, these guys can be very difficult to feed unless you buy them specialized filter feeder foods. They can clear up green water very fast, but will afterwards starve to death and cause an ammonia spike.
Only plants (including algae) can remove nitrates from the water as part of their metabolic processes; no animal can do that.