But "naturalisation" can only relevant in a situation where someone is not a "natural born citizen", but is to be given citizenship.

It is quite distinct from registration, which is basically a method of recording certain facts - the facts being that someone has been born at a particular time and place with particular parents. It is surely by virtue of those facts that the person is a citizen, not by virtue of the registration, which is just a convenient way of proving them.

As I said, the lawyers could have a great time with this.