Salieri's confession itself was never anything more than a rumor. Following his delirium and suicide attempt, it was said that he confessed to a priest that he poisoned Mozart, but there is no such written note or testimony (I suppose there couldn't be, since priests don't reveal confessions). Again, we can't prove with mathematical certainty that he didn't confess, but given that he had many mutual friends with Mozart and that they defended him from these accusations throughout this period, we have no reason to believe such a thing.