Hmmmm.. in a whole bunch of years of doing API testing and helping others with them, you're the first to bring this up. I dug out my own booklet, which is several years old and I see what you are talking about.
My first thoughts are that ALL FW tanks have some salt in them. Unless folks are using distilled water or RO water (BTW, neither of which will support life very long), there is going to be some salt in the water. Since the page doesn't say how much salt, I'm almost wondering if that is either some kind of slip up that nobody ever caught until you did... or if it's some kind of marketing ploy to try and sell API Aquarium Salt... which is just plain salt at about 10 times the price of plain salt bought at a grocery store. Without some definite reply or information from API, I would say that ALL FW has salt in it already so the test kit should work. I've seen folks compare the API kit to other kits and I have compared the API kit to the Tetra-Laborette kit and came up with very similar results. BTW.. those two kits are both available at Walmart.com for around $15.00 each with free shipping to your local store... best bargain I've found on a master test kit!
I did a Google search of that quoted sentence and found a couple of other hits so you're not the first to ever notice that sentence. There is even this old 2007 thread from this forum... but the OP never did post any reply from API.
http://www.aquariacentral.com/forums/showthread.php?p=783194. Every other hit that I looked at were to actual product sales pages where they had the instructions re-printed on the sales page.
So, all that said, I would say that the fact that all *normal* FW has some salt in it, the results should be accurate... unless API says there is supposed to be some kind of elevated level of salt... then they have some serious 'splainin to do (in my best Ricky Ricardo voice).
