Actually, it was.
Think about it. "Doc" Smith could be forgiven for postulating iron as a source of the insanely huge amount of energy needed to bypass Special Relativity, because the pre-Lensman version of Triplanetary predated the "Chicago Pile-1" experiments in a squash court under the stands at Stagg Field (by nearly a decade), and even then, nobody knew enough about nuclear reactions to understand that iron cannot produce a net release of energy by fission or by fusion.
But "The Man Trap"? Common salt is an ionic compound, consisting of sodium cations and chloride anions. The sodium cation and the chloride anion are two of the most soluble ions known, and they have a strong affinity for each other. While a real-world life form that uses common salt is easy enough to find (there aren't many that don't), one that metabolizes salt, and takes it permanently out of the environment is preposterous. So is one that can quickly (and painfully) extract it from another life form. Likewise, a life form that can simultaneously present three different appearances (one radically different from the other two, and all three radically different from its natural appearance) to three different observers wouldn't fool sensors, while a true shapeshifter would only be able to present one appearance at a time.
Maybe the M113 creature was feeling...
salty at being the last of its kind.
