Because an auto-pen was used and Biden's mental competence is in doubt then any prosecution should be able to raise enough question in the Court's mind as to whether Biden did indeed authorise this auto-pen use that the defence would need to aduce evidence that it WAS authorised. This would likely need Biden to take the stand and his further mental deterioration would likely prevent this or make it damming for the defence.
this guy said what I was thinking so perfectly, i'll just share a screenshot with you.