Actually, many of the cultures that include references to salt and purification don't have much else they could mean by "evil spirits". Evil spirits is an exceptionally clear term, even if what they attributed in the past to evil spirits might well be what we would think of bad luck. An analogy wherein multiple translations casts an event (especially a fictional one) in a different light doesn't mean that salt isn't linked to evil spirits.
A common superstition is that spilling salt can attract evil spirits, thus necessitating throwing a pinch of it over your shoulder to ward them off. To quote a source I found when I did a little reading (because I get bored), "In Buddhist tradition, salt repels evil spirits. That's why it's customary to throw salt over your shoulder before entering your house after a funeral: it scares off any evil spirits that may be clinging to your back."
Stating that Supernatural thought up the link between salt and spirits is simply untrue. It may have instilled it in the public conscience, or represented it in an exaggerated way to aid the narrative, but the link in warding off spirits is there.