Scientists can only work with what they find. The article you link explains that they can make those inferences because the geology of Mars allows them to see a more reliable history in its rocks, whereas the tectonic acitivty on Earth has "erased all evidence of what happened in the first 500 million years of our planet's history." Thus, they can't do more than speculate about whether there was ever that much water on Earth in the past.
Also, it's pretty awesome that there's a Professor Martin Bizzarro out there. That's a Bond villain in the making if there ever was one.