"Scientists have confirmed chemically..."
This seems to sum it up pretty well. The constituents of Mars differ from that of Earth, the Moon, or any other rocky entity that we scan, test or study. We know the Moon was once a part of Earth because it shares similar chemical makeup to that of Earth. That being said, the eons that have passed, and more precisely, the organic alterations that have taken place on Earth have modified the chemical makeup of Earth's crust enough to differentiate between a sample collected here or there.
While it could be coincidence, you could equate this to finding a MacDonald's cup across the street from McDonald's. Could it have come from another McDonald's? Sure, but it probably came from the closest one, and it sure as hell didn't come from the Burger King down the street.