Water? In MY upper mantle?

[Drashner1]

However, if this turns out to be true, this means that terraforming just got harder - a lot of hydrogen that went into the earth's water would have come from terrestrial sources rather than in commets.

Not necessarily - if Mars has a lot of deep water, it might be pumped to the surface to assist in terraforming. And even if a lot of Earth's water is 'local' instead of from comets - that doesn't preclude using comets to bring water in for terraforming purposes. Or mining ice moons and sending the ice to the planet to be terraformed.

It would seem to mainly mean that the possible options for terraforming might change up or even expand a bit.

Todd