It's nothing but speculation. The same as with the elegance corals months back. Nobody knows what is going on and they are throwing out big words and making assumptions.
The before and after photo proves nothing. The only thing it does show was a mass infestation of stars on the clam itself which would explain why the clam was unhappy.
FW dips are not CureAlls. The problem is this... "my fish/coral is looking bad, it has stuff on it". Typical response, FW dip it! Dipping is dangerous. Saltwater and freshwater have different barometric pressures. In essence, when a dip is preformed, cells "burst" under the different pressures. Most times the dip is not long enough to do damage on a grand scale, but it does do damage on a small scale along with stressing the item being dipped. God only knows how painfull it might be to fish themselves. Granted clams don't "think".. it's still not a normal nor wise thing to do.
Perhaps large quantities of Ultraviolet light may have had something to do with it's untimely death? Maybe kalk or vinegar additives? Maybe rapid changes of temp, light or salinity? Who knows. I don't think anybody will.
Sorry for your loss. I just think this whole "pinched mantle" thing is bogus. I have had over 45 clams in my reefing life, not one has ever had a disease, died or otherwise perished from some unknown factor. None have ever died. Maybe I am lucky? Who knows.
JMO. YMMV.