Who cares? Why does the game have to fit within canon? Canon isn't even a real thing. It's just something nerds obsess over.
Why even bother writing in an established setting with established characters if you're not going to respect anything about it?
Citation please! As far as I remember according to the at-that-time-canon but horrible Gehenna novel it would still have been a possibility that Caine impersonated the cabbie.
It's one of those memes that have apparently been lost to time. Regardless, Caine's personality and motivations in Bloodlines outright contradict the description of Caine as given in the Gehenna book (not the novel, the one with multiple endgame scenarios). They provided players with multiple (admittedly lousy) "canon" endings, but Caine's personality is fixed. Troika botched it.
That's not really how this works.
It can exist as your headcanon, just like Deckard being a human is my head canon, but officially, the guy's a replicant.