Typically they are – for two of the same reasons, first being that most of the “salt alternatives” in use, the original “salt” in this case being sodium chloride, are also chlorides (potassium or calcium chloride, usually) and it’s that chlorine ion that’s corrosive. They also all turn the meltwater into an electrolyte, forming an easy electrical connection between the various metals in your vehicle’s parts and dramatically accelerating galvanic corrosion.

Technically any compound composed of positive and negatively charged ions that balance out to a net neutral is a salt, chemically speaking, and by definition they are compounds, i.e. held together with weak ionic bonds via their electrostatic charges and not molecules held together with strong covalent bonds. This means they like to liberate their constituent ions easily, allowing whatever-it-is they’re composed of to readily react with something else.

TL;DR: Pretty much all salts, not just sodium chloride salt salt, are corrosion promoters.