If the theory is correct, then Earth was created by a collision of two hunks of rock, neither of which was the Earth in any meaningful sense. I'd imagine that
everything we have is substantially different from either of the original masses: different surface (because the old ones were utterly scrambled), different orbits (because it seems unlikely that orbit.a + orbit.b == orbit.either_one), different compositions (because TFA says they were made out of slightly different stuff).
It's not like the Earth was chugging along happily until something came along to disturb it. The Earth as we know it was created from other things at the moment of impact. Both young worlds were alien, and so neither one really is.
On a marginally related note, I know the solar system was way too young at the time for there to have been anything you could reasonable call "life". Still, I think about what if there was life on either or both of those bodies, and it was intelligent enough to look up into the early, hostile sky and wonder what that brightly glowing, daily growing circle in the sky was. Were little animals awakened in terror at the sound of their world ending? Did an ancient family hug each other one last time and close their eyes as the tidal earthquakes began?