Where, exactly, in the conversation did Urianger ever say, as you stated, that Light "contains none of the elements that make up life", and that "Dark is all the elements that make up life"?
All he says is that there are six elements according to the Eorzean (and implicitly Norvrandtian) ideas of aether, plus two polarities that all elements are aligned to. What we call "Astral" is what the people of the First call "Darkness", and what we call "Umbral" is what people of the First call "Light". "Astral"/"Dark" is attributed to activity, and "Umbral"/"Light" is attributed to passivity.
In fact,
I made an Imgur album with screenshots of the relevant dialogue.
There is
literally nothing in that scene I can see which relates to your assertion that Light "contains none of the elements that make up life". If you are trying to argue the polarities placed on that elemental chart, you would also be claiming that
Water (which tends towards Umbral/Light) is not "an element that makes up life", much less Earth. (Note that the very first elements we attempt to return to the world in the Eden raids are Water and Earth.)
So, once again, citation needed for your claim, please.