The source where Martin claims it's not Benjen is actually an original manuscript of ADwD being displayed at a university, where the editor in the margins questions, "Is this Benjen? If so, why doesn't Bran recognize him?" to which Martin responds "NO." As in, not only is it not Benjen, but his body is not recognized by Bran either, who spends quite a bit of time with him in the books. Not to mention the other contextual clues that Coldhands is much, much older than Benjen's disappearance.