Traitors are obviously allowed to accuse each other of being traitors in the course of the game and using the same tactics as the faithful, but they can't say "I know he's a traitor because I am one" as they leave. It would effectively bring the game to an end. None of the traitors in the Aus or English version have done that, which clearly suggests it's against the rules. But Kieran sailed extremely close to doing just that, which effectively meant that he used inside knowledge to get personal revenge and was unfair to Will. None of the other traitors in either series has done that, so I suspect they rightly believed it would be wrong and against the rules. It made for quite a good ending, and I suspect the producers didn't stop him for that reason, and maybe because they decided to give him the benefit of the doubt about the interpretation of the rules, but I bet they'll spell it out in future.