This was covered when Jason Knauf’s gave evidence to support ANL’s defence of Meghan’s claim for breaching her copyright by publishing extracts of her letter to her father.
Both Meghan and ANL had approached Knauf shortly after the claim was first filed, and each asked him to give evidence to support them (this is completely normal, the court is simply interested to hear evidence from people about the facts that they have personal knowledge of, and it does not consider a witness to “belong” to one side or another). Initially, Knauf’s response to both sides was to say that his position was "strictly neutral" and he did not wish to have any involvement in the proceedings.
As the case developed however Meghan gave witness statements and made submissions to the court that were misleading about what Knauf himself had done (she said none of her staff had done any fact-checking of Finding Freedom, and she also said she’d never authorised her staff to have any involvement with Omid Scobie and the book). Meghan won her case in the High Court, and ANL appealed.
Knauf knew the statement made by Meghan, Omid Scobie and her legal team were untrue, so he made contact with ANL to let them know that he was willing to provide a statement from them to use in the appeal. It was at that stage that the comment was made that he regretted not giving evidence during the original High Court case.