"One of the challenges to the longstanding theory of the collision, is that
a Mars-sized impacting body, whose composition likely would have differed
substantially from that of Earth, likely would have left Earth and the moon with
different chemical compositions, which they are not."
HMMMmmm...
Then this hit-and-run body would have EQUALLY mixed with Earth stuff and proto-moon stuff,
since both bodies are so similar.
or...
Every bit of the Unknown Collider would have vanished off into space somewhere.