Earth would definitely show a scar. The Erosion argument is over played. And where as every other planet has evidence of a large impact (Mars and Venus) Earth is the only one that has plate tectonics where as scientist believe that the impact some how terminated plate tectonics. (Likely melting the entire surface)
Moon creation Theory places the impact at 4.5 billion years.
They project plate tectonic 3.6 billion years ago. (900 million gap)
Earth's oldest Sedimentary rocks are 3.9 million years old.
Oldest Rocks on the moon are 4.8 million years old.
Moon rocks have evidence of being molten. (the complete surface)
Earth's rocks have no such evidence. Our rocks show that Earth earth's heavy elements aren't shifted toward the core. They are still on the surface sprinkled randomly just like accretion would allow. If there was a massive impact then Earth really shouldn't have plate tectonics either and should have a cleanly separated density of materials. Nor should it have any water.
That's why they came up with the cometary bombardment theory to bring water to Earth but still no other planet in the inner solar system has that characteristic either. Essentially the moon should have water in permanently dark craters in ice form.
The Great Collision Theory just doesn't add up.