I think Dr. Morin is saying that a nitrate test only shows
nitrate (or nitrogen from nitrate and then you convert to
get the nitrate value) and not other forms of nitrogen that
are not from nitrate such as Flourish Nitrogen has. Half
the nitrogen in Flourish Nitrogen is in the non-nitrate
form. So if you tank has no nitrate and you add Flourish
Nitrogen and measure 5 ppm nitrate, then the amount of
nitrogen you added is equivalent to having added 10 ppm of
nitrate.
If your tank has 8 ppm nitrate and you add Flourish
Nitrogen and measure 13 ppm nitrate, then the the total
nitrogen you added is equivalent to 10 ppm so you could
treat a measurement of 13 ppm in that case as 18 ppm.
Hope that helps more than it confuses,