This is an excellently put objection - putting it this way makes it clear just how strong the objection is. The likelihood ratio to me sounds like it should be more or less T/F, where for the sake of conservatism T might equal .99 and F might equal .01. If we knew for a fact that there were aliens in our radio range, wouldn't this item of evidence wash out any priors we had about them eating stars? We don't see the stars being eaten!