It’s unclear to me what they are saying. Are they saying deuterium was the water native to the ancient Earth and the H2O came later from asteroids and other objects?
Most water contains one atom of oxygen and two atoms of hydrogen: H2O. But some water contains deuterium instead of conventional hydrogen. It's a heavier version of hydrogen that has an extra neutron at the center. When scientists find the source of Earth's water, they expect it will match the fraction of deuterium that scientists observe in Earth's oceans today. But measuring that is tricky to do without physical samples like those from Itokawa.
The ratio of regular water to heavy water doesn't match, but A) the asteroid has no atmosphere, so weathering in space (a real thing) has altered it, and B) not all the water was delivered from space, but if half of the water came from impacts, the ratio averages correctly. :^) A better test would be to snag those house-sized chunks of ice that continually hit the Earth's atmosphere and shatter at (usually) very high altitude, and see what the ratios are.