Yeah but FIFA don't talk about what citizen status they hold. They specifically say 'if they are born'. If it said 'if they somehow hold a citizenship by some contrived manner at birth' then fair enough. In the above, they are born in the UK. Guess who they represent?
I think you are putting in too much time when its black and white. At the end of the day, they have to be born somewhere. If its not NZ and they don't meet the other criteria then they have to turn 18 and wait 5 or get an exemption. The end.
What is interesting though is that Article 6 technically applies to NZ because of the Cook Islands - and that does say he must have lived there for 2 years, or been born there, or have a parent or grandparent who was born there. This hypothetical player might not be acquiring a new nationality but he would fall under article 6 and wouldn't meet requirements to play for either NZ or the Cooks. Whereas if he was say, Australian instead of kiwi, and all other factors were the same from my reading he would be eligible to play for Australia because as far as I',m aware there's no other FIFA member which shares Aussie citizenship. Bizarre. I suppose you could get an exemption for that too though maybe.