According to the website eburacum45 posted

"1. Samples from Mare Imbrium and the Ocean of Storms brought back by Apollo 11 and Apollo 12 are about 3.5 billion years old, which is comparable to the oldest rocks found on the surface of the Earth."

If there are rocks on earth of equivalent age as the moon rocks then that allows for the implication that the two share a common source, i.e. the moon was ejected from the earth's crust.