Ian, I don't see it. Do you have a link? From what I have read is that the only thing you need to worry about is chloride interferance. Most people use RO/DI, so that shouldn't be an issue.
Having worked in the Water Wastewater indistry for over 30 years I can say there is not a good reliable, repeatable portable nitrate tester on the market. I used many and all have their problems.