While I have a basic understanding of what spectroscopy is, it doesn't mean it's infallible. Simply put we know how to examine materials based upon spectroscopy over very short distances through known and understood conditions. To this point we've only been able to prove that spectroscopy is capable of determining composition of items over a couple meters to a couple million miles. When you apply this understanding to a example that is light years away with who knows what between us, there is a lot of room for the rules to change.

Furthermore we don't know what kind of compounds might exist on such a distant planet. Conditions, molecules, elements, energies may (probably) exist that we just don't know or understand that could contradict our understanding of material science.

I'm not saying that this "finding" is wrong, just that it's a interpretation of something VERY far away that more than likely will never be proved in our lifetimes. To call it conclusive is rather strong.