As I said in another comment, you may be reading too much into the statement to the press as opposed to the actual paper. It's more likely that the author quote

As I said in another comment, you may be reading too much into the statement to the press as opposed to the actual paper. It's more likely that the author quoted as saying that something could be "ruled out" meant, more precisely, that the possibility of contamination is very small, but non-zero, and that the overwhelmingly more likely conclusion is the existence of said compounds.