I've always wondered what percentage of water molecules on the earth have always been water molecules, i.e. since they became H20 how many have been separated by chemical processes and later recombined back into water. How you estimate that is beyond me.
No, that's how you estimate something completely different - the age of the hydrogen atoms that make up the water.
I believe that the GP's point is something like this: It says "the water is older than the sun", but what it really means is "the hydrogen atoms in the water are older than the sun".
The subsequent paragraphs explain how they go from "age of hydrogen atoms" to "age of the water." If you want more detail than is in the article (which is understandable), then you'll have to refer to the published paper.
i guess it depends if it came before or after the formation of our moon. If the mars-ish sized planet wacking into us theory is true, probably not much if it came before and just re-condensed after the collision.
