I don't know anything about this salt mix, but I was just thinking that a two part salt mix would be more stable as the calcium and carbonate would not recombine in the bucket in the presence of moisture. This is not an issue with unopened buckets, but once open in a humid environment there will be problems.
If I were to make my own, I'd probably make something similar to IO, but with even lower calcium and alk levels, and more normal levels of magnesium. Then just have the user add pure calcium chloride and baking soda, each pre-dissolved in fresh water. A mix like this would be superfast to dissolve. Problem is it would be way too easy for people to screw it up and then complain how you killed all their fish.