This is starting to get abovce my head but are you speeking about a single tissue or all tissues when you cal the pont of saturation? I dont believe you ar saturated when yo hit NDL. By the definitions you are citing,,, to come out of saturation you just go deeper brvsudr your tissues can now take on more. Something for me is missing from your position on this.
If you go deeper you are no longer saturated. Henry's law.
I don't like 'saturated' as a term here. I think 'in equilibrium' is better.
Different tissues get saturated at different times, the rate at which the gas is transported varies. Tissues are modelled with half times down to about 5 minutes so it is likely that on any typical dive some tissues are saturated. For example blood gets a pretty good exposure to the nitrogen in the lungs pretty fast.
Whatever tissue you consider, given a lower ppN2 due to nitrox in the lungs it will end up with less nitrogen at any given time into the dive than had air been used.