Nitrox reduces the percentage of nitrogen in a given mix at a given depth.
No. The percentage remains the same.
Regardless of "given depth".
Actually, I think that you may be misinterpreting waterpirate's statement. Although the term "nitrox" is inexact (it could technically refer to any mix - hypoxic, normoxic or hyperoxic), I assume that he is using it to refer to mixes with more than 21% O2 (i.e EAN, or enriched air nitrox).
I believe that he was saying that it reduces the percentage of nitrogen as compared to air. "Reduces" is probably not the best way of phrasing that, though - you could say that EAN has less nitrogen per unit volume than air, but the concept is the same. For example, EAN 32 has 68% nitrogen by volume, as opposed to the 79% nitrogen in air. This is true at the surface or at any given ambient pressure, according to Dalton's law of partial pressure (each gas compresses equally for all practical purposes).