Note that the very interesting graph near the end of the article actually displays element abundance in the solar system, not in the universe. (Found that out when looking it up in Wikipedia.) The article misrepresents this!
The cosmological principle works when considering a large enough scale. A solar system of a particular type of star, even though that star is pretty average and unremarkable, is not really large enough.
While water will break down at high temperatures I would find it surprising that 1st and 3rd most common elements in the whole universe, that readily combine to form water would not result in substantial amounts of water.
It's probably like saying we shouldn't find H2 because "nothing says they are found bound together"
Most atoms- and certainly most hydrogen- are in stars, no? Not exactly places where water "breaking down at high temperatures" (really, never forming) can be ignored.
You provided a conjecture based on incomplete data as fact, he stated that it's conjecture until additional data to support it is there... the burden in this situation is not on him.
> You provided a conjecture based on incomplete data as fact,
No, I never said it is a fact, it is, as you pointed, a conjecture, and I provided a source (ok, wikipedia) for the amount of water in the universe
If you or him knows of a reason for why the most common elements in the universe that combine easily wouldn't result in an abundance of their combination I'm all ears.
(Which doesn't mean the amount of water is going to be near the amount of H2 or just the amount of Hydrogen present in the stars)
tl;dr: Oxygen
But it's worth reading the article to learn why.