As a clarification, again: Production merely means built in numbers and identical. The production line means that there are workers who are dedicated to building them... not whether they are on a line or belt per se (otherwise, many supercars would not qualify).
It could be argued either way. It's just that to qualify the fact that "other manufacturers won't care" and pointing to the Z-Tune just isn't a solid case. If Nissan chooses to consider it a production car, it is. It was built in numbers of identical lots and sold to the general public.
The fact that one of the raw materials was an entire car instead of a couple hundred kilograms of raw steel, rubber and etcetera doesn't change that fact. Thus, it was manufactured, technically, even though it was, in reality, remanufactured.
Of course, that interpretation opens to question whether "tuner cars" can be considered "production" if more than one kit is produced... but most tuners don't bother to sell the entire vehicle, they merely sell the kit as screwed onto the car. One notable exception is the JohnCooperWorks version of the Cooper S, in which the kit is a dealer option that's sold at the dealership with a manufacturer warranty... although what the registration papers say, I don't personally know. While RUF is a manufacturer in their own right, they can and will rebuild a standard customer-owned Porsche from the ground up to one of their production specs.
The vague definition of "production car" is often used by the big manufacturers as a shield to protect themselves in terms of bragging rights from those persnickety upstarts who just happen to make sportscars that are, unfortunately, much, much faster than any Porsche, Ferrari
or Nissan... like the Radical or the Donkervoort... both of which hold track records that manufacturers and publications conveniently ignore everytime some new "production car" sets a new Nurb record...