Dynamical origin of Theia, the last giant impactor on Earth

So the boil-down on this is "Here's a theory that says about 5-10% of Earth's mass was mostly carbon and came all at once, like if Theia was mostly carbon and we got hit by it, so we did some simulation and the idea Theia was mostly carbon isn't ruled out by our current understanding of how our solar system might have formed?"

If so, cool. It's a wise step to check the hypothesis to make sure it isn't immediately contradicting what we already understand.

No, the 5-10% mass was not mostly carbon. It was also mostly oxygen with smaller amounts of iron, silicon and magnesium, like in any parts of the Solar System where the gravity is not so strong as to retain most of the hydrogen and helium.

That mass has come from bodies which have collided with Earth and which had a chemical composition similar to that of "carbonaceous chondrites". (That is a kind of meteorites; most of the Earth had a composition similar to "enstatite chondrites", another kind of meteorites.)

The carbonaceous chondrites are not made of carbon, they only have more carbon than the Earth and similar planets, because in the early Solar System the carbon was present mostly as carbon dioxide, which is volatile so that most of it does not condense into the planets at the higher temperatures of the zones closer to the Sun, where the inner planets have condensed, but only at lower temperatures, farther from the Sun, where it reacts with metallic oxides, forming solid carbonates, like limestone.