SIMILAR isotopically, different CHEMICALLY
The Moon is drastically deficient in volatiles compared to the earth, yet shares the same isotope ratios of elements that should have been fractionated in a volatile environment, implying that the volatile depletion occured very rapidly, in geologic terms
The Collision has some odd aspects - for instance, Theia and the Proto-Earth must ALREADY have been isotopically similar (hence the trojan planet concept).