Keith I think the argument revolves around clear unequivocal proof. The chemical has been identified (we are told) Experts tell us it is not a difficult synthesis. It is pure. Therefore any telltale contaminants that might identify location just are not there. No one can prove who made it!
Therefore this presents a problem. Many people believe in God. Are they correct or misguided? How many wars have been started as a result of deliberate "lies"
We deliberately carried out an act of war on the basis of if, buts and maybe's. Might be good enough for you, but not for me. If people are going to killed in my name I do not think it unreasonable to demand incontrovertible proof, especially if I am expected to take the word of MP snake oil salesmen.