The point is, that within the game, the taxi driver is Caine, because writer said that it's Caine.
Although I believe this myself, where did a writer actually say it? The internal links are unclear on purpose, like only the Malks recognize him and we all know that they are crazy. The sound folder being called Caine is not a statement of the writers, only another hint.
Deckard is replicant because the author said he is replicant and you can't change that.
Where and especially when did Scott say that? I always had the impression that Deckard was a human during filming and Ford confirmed he played him as one, and then afterwards Scott thought it might be cooler if he was a replicant and retconned it for the director's cut. Similar to that Vader wasn't Lukes father in A New Hope and Leia was not his sister in The Empire Strikes back, although Lucas latter claimed that he always planned it that way...