In the first few hours after publication, that would be a reasonable question. But since it's been a full day, and Seymour Hersh hasn't stepped in to say "Hey, that's not me", at this point it's quite reasonable to believe that he wrote it. It seems unlikely that the piece generated a full Whitehouse response but somehow Seymour, his agent, his publisher, or any of his close friends haven't yet issued a denial of authorship. Not impossible, but not the way I'd bet.