Technically correct. The problem with the salt (NaCl) is that it is hygroscopic, meaning that it absorbs water from the air and becomes damp. Since if it is mixed with the baking powder, this starts the acid - base reaction.
The powdered sodium bicarbonate (base) and (usually) cream of tartar (acid) will not react if kept dry even if mixed, but the salt is the culprit which makes them react with each other.
If you're making a dough, you will usually add some water, which makes the reaction kick into high gear, making lots of Carbon Dioxide gas immediately. You want this to happen.
If you mix these ingredients and just let them set in a jar, they will slowly react and eventually you will get a mix that will not rise even when you add liquid.
Hence my advice; don't mix these until, at most, a couple of days before you plan to use them in a recipe. You can put the mix in an airtight jar and it (the salt) will take whatever little moisture is in the air left in the jar and start the reaction... Slowly killing your mix.
This is one of those rare instances in which the expiration date on real Bisquick(r) actually means that it expires. If your box of Bisquick(r) won't rise, you can add your own baking powder and cream of tartar and it works again.
I'm not just a cook and a chemist (BS Chem. 1973), but I play one on TV in my spare time....
NOT