Although the AAIB has a lot of in house experience, they rely on outside experts to analyse and test specific components and also to provide expert commentary as to human factors.
Those same experts can also be employed by the police or any other party to provide expert testimony in a court case.
Just because the AAIB report cannot be used to assign blame, that does not mean that the contributors to the report cannot be compelled to give evidence in a criminal or civil trial. That includes members of the AAIB team if a judge rules that their testamony is necessary for a complete understanding of the matters being considered.