Implying defamation rather than admitting to it IMO.
As I see it the McCanns complained that Amaral's actions damaged their reputation. Everyone who sues for defanation believes that something someone else said damaged their reputation so how is it proved?
One way is to prove that a false statement has been made. The courts examined Amaral's book and found that it contained verifiable facts from the investigation, so no falsehoods there.
What he did was express his opinion about those facts, so was his opinion unlawful in some way? It wasn't, becauseit was the same opinion that the investigation had reached using the same facts.
The defence was left with trying to prove that he wasn't allowed to express his opinion. They offered arguments designed to demonstrate this;
His retired status.
The McCann's 'cleared' status.
The courts ultimately rejected their claims and found he was entitled to express his opinions.