I now remember the link you posted.
Even if API salt, as an example, isn't 100% NaCl then what measurable effect would it have on GH/KH...enough to even worry about? You said it has "considerable" amounts of other ions, but in reality not enough that the manufacturer is obliged to make known on the MSDS. Keep in mind that regardless it's still raising TDS which is pretty much all that matters to the fish anyways. Hence why I said it affects osmoregulation.
The overall point still stands, you do not NEED aquarium salt in a FW tank. That would even be confounded by the point that theoretically it COULD recreate brackish conditions. Besides chemical analysis there are loads of people who do not use it and are successful. I will admit I may have misspoke when saying "100% NaCl" but at some point if it doesn't warrant mentioning on a MSDS then why even worry about it at all? Bottom line is that anyone who has a FW tank should not need to add it on a regular basis to achieve the desired results.