The Thursar do not know that Yggdrasil exists. Its glassy gold is to them, as to many of us, invisible, and thus they laugh at its invocation. To them, only the chaos of monstrous matter, visible before their eyes, exists, and they know not, therefore, the cradle and womb of that chaos, which takes it up and weaves it into a living context.
The living order at the heart of matter is not immediately evident to purely empirical eyes. Such a notion is easily scoffed at, particularly by those for whom only the effects of brute force are real. (Think of how much "force" is a basic concept in our rudimentary physics!) But there are those who are able to see with eyes of meaning, and know that what they see through those eyes is as real as what the physical eyes see.
You can't see Yggdrasil. You can't point to its branches. You cannot show its trunk. You cannot prove its deep roots. Yet it is that totality of life and meaning, which encompasses and yet transcends the biological and material, for which the Gods train to fight and protect. It is that which fully matters.
It matters because it is important ; and it also "matters" in the sense that it exhudes as an epidermal shedding from its deeper flows that material world of flesh and bone and bark and stone we know so well. Thursar look at the shed husks and conclude it is all.
No wonder they are so hungry. No wonder they would eat the world. For they feed their souls on the tablescraps of the Great Feast, and bemoaning their starvation, brutally assault the world to feed that endless gnawing.
There is more. It is alive. It cannot be seen with physical eyes.
This is what we teach. This is the essence of what the ancestors passed on. This is the center around which even Gods are peripheral. The cosmos is alive, and we only follow our own Gods' example when we do worship a tree.