On the side of the formula tins and also on their website it recommends their ready-made formula because it will save you from having to bring out containers of powder and separate bottles of cooled boiled water. IMO this implies that doing this would be an acceptable alternative. I know they are out to make money and all that but do they not read the guidelines? Or have any part in them?
I am especially confused as if they suggested following the guidelines that would be more faff than the cooled boiled water/powder and make their expensive ready-mixed stuff even more of a handy alternative 
Looking forward to their response, but wondering what to do if they basically say they think it's fine.