And since he isn't really the Doctor (and never really was), it doesn't really matter, IMO...
That's debatable though, isn't it? The language used is somewhat nebulous and even contradictory.
He's a 'facet' of the doctor (I believe the word used was 'aspect'). This implies he is a part of the whole.
He is also described as a 'potential dark version' of the doctor, and the novelisation of
Trial.. describes him as the " penultimate reincarnation... Somewhere between your twelfth and thirteenth regeneration" which could be interpreted as implying he is the thirteenth doctor (i.e the time 'between' the 12th and 13th regenerations is the time period of the 13th doctor).