Yeah, the article is really frustrating because it only addresses that question in a roundabout way. It says "The Precambrian Canadian Shield that stores vast m

Yeah, the article is really frustrating because it only addresses that question in a roundabout way. It says "The Precambrian Canadian Shield that stores vast mineral deposits is 2.7 billion years old, and once formed an ocean floor." - so I guess this water got trapped while the ocean floor was subducted or overlaid with sediments or whatever happened that brought it to its current depth?! So it's not the H2O molecules, but the whole liquid which has stayed there for that amount of time. Referring back to your original question, it would be fascinating to find out what percentage of the water molecules in circulation on Earth today date back to the time when water first condensed out of the atmosphere (IIRC ~ 4 billion years ago) - but I'm not sure if that's possible.