I think you're reading too much into the phrase "can be ruled out." I suspect that the leader author was saying "Contamination is possible, but based on our pro

I think you're reading too much into the phrase "can be ruled out." I suspect that the leader author was saying "Contamination is possible, but based on our procedures, test methods, etc, the overwhelmingly larger probability is that we did detect <...> in the samples, and that the detection was not from pre- or post-contamination." Could the paper's conclusions be incorrect? Yes, I think everyone--the authors included--will agree that "something could have gone wrong." But pointing out that "something could have gone wrong" is probably not a new idea to most readers.