Drop checkers have been out for many years, but adding the 4 dKH carbonate based solution with its included bromothymol blue solution is rather new.
That is why your directions say to use aquarium water.
You can't use aquarium water even if it is 4 dKH either. There are more than just carbonates that add KH to the water than carbonates. Phosphates, other acids and bases add to the KH mix too. This invalidates the pH/KH/CO2 relationship. This relationship is only good is the water uses carbonates which is where the 4 dKH carbonate based solution comes in.