Why is the evidence actually favorable to the defendants, as explained in Chapter 7?

The only evidence for the main charge comes from the testimony of the sole survivors (a mother and daughter) and the daughter's book of her experiences as a prisoner. The narrator (Michael) reasons that a competent defense would have been able to show that it was impossible to say whether the defendants were the actual ones who had done the selections, because the witness's testimony was not precise. Furthermore, both of the survivors had been inside the church, and therefore could not testify as to what happened outside.