It was a tongue in cheek answer. The way the prosecution deal with it is to commission their own experts to review the available/admissible evidence and give an opinion. The prosecution were able to use film of the incident, NATS radar data and in some instances non AAIB witnesses who had provided opinion to the AAIB ( i.e. medical witnesses, the test pilot (who had been commissioned jointly by the police and AAIB) etc. Because all of this was available independent of the AAIB and not protected material. The witnesses essentially gave fresh statements.
Ah, Hoyle! Another tragic example of what happens when a loop is poorly flown!
In fact no, that was the allegation made by the prosecution but denied by the defence. The defence produced evidence that the aircraft was not flying a loop. In reaching their not guilty verdicts the jury were not satisfied that a loop was being flown.