Indeed. The sort of point that ought to have been raised by the Guardian. But still it raises the question, why do (any) comets have heavy water on them as opposed to normal water? Answers could be very interesting.
Deuterated water requires more energy for phase changes to occur. The result they obtain could simply be a result of losing regular H2O to space at a slightly higher rate for billions of years.
Most of the heavier elements in our solar system condensed from the accretion disk into the rocky planets, nearer to the sun than the gaseous outer planets. That's why it's so strange to me that we'd find higher abundances of heavier water out in the Kuiper Belt or Oort Cloud than there presumably was/may be on asteroids, which are much more near to the sun. ...I love a good mystery!