Well, its because the credibility of the evidence is not predicated on its complexity. If SETI were to announce that were not alone because it had detected a signal, it would be on the basis of artificiality.
this is a gob smaker. so the signature of intelligence is its artificiality.
Exactly so. An artifice is something made by a person, or in this case an extraterestial.
Dr. Shostak was very clear that they are not using complexity as a measure of intelligence. They have no need to assign some outside intelligence to something that is obviously natural and follows the natural laws as we know them. We do not observe any natural process that could produce a cosmic symmetrical cube so we can safely infer if we observe such that it was produced by an intelligence. This is artificiality. In other words, artificial with respect to nature.