They played with idea of Deckards lack of humanity all through out the film. Rachel even saids "have you ever tried the Voight Kamphf test on yourself?" But, the screenplay was never about a literal interpretation that Deckard was a replicant. Deckard was a replicant in terms of his soul. The job of a Blade Runner had robbed any sense of his human self.... Scott, stupidly, thought it was a great idea to be "on the nose."