There's even a possibility that as much as a quarter of the Moon's water came from the widely theorized Theia impact with the Earth around 4.5 billion years ago, which would have shared material both directly and through asteroids.

The problem with the asteroid concept, as you might guess, is a relative dearth of information. Scientists have only examined 2 percent of the Apollo samples so far, and they're all from the near side of the Moon. It's possible that further studies (especially from the far side) will tell a different story. Thankfully, those missions are coming -- the theory may get additional support in the years to come.