I'm not sure that showing that XYZ can't make something water-like is any more helpful than just pointing out that there isn't actually a Twin Earth. Yes, it was supposed to be a counterfactual thought experiment. Oh noes, the counterfactual doesn't actually obtain!

And showing that particular chemical compounds don't make water, doesn't entail that there is no XYZ that makes water.

And as army1987 pointed out, it could have been "cat" instead of "water".