I'd say that's not strictly true,mostly because no salt substitute I've ever seen can get the same taste anyway.
"Real" salt is sodium chloride (and maybe iodine, or minor impurities if it's sea salt). Most salt substitutes are partly or mostly potassium chloride. This is technically a salt (chemically speaking) and does have a similar salty taste, but it doesn't taste quite like sodium chloride, and can be bitter or metallic-tasting. Some salt substitutes add herbs or other things to help make the potassium chloride more palatable. Sounds like your has some "regular" salt as well, if it's got a significant sodium content.
So while you are right that there's likely a "catch," it's more likely to be that your salt substitute will never taste quite right, or will be bitter, and less likely that you'll be piling loads of extra on in an effort to get the right saltiness.