The issue isn't that GJ defended his investigation. The issue is that he gave an opinion on a current POI. A positive reference, no less.
A person's character is a huge part of whether they are found to be credible or not, and the main part of convincing a jury is to cast doubt on the defendant's credibility.
Should a POI be prosecuted the DPP following an investigation by police, it is beyond comprehension that the POI's credibility could be bolstered by comments from a detective that worked on the case.
It's the equivalent of a defence lawyer walking in and saying to the judge his client is a bit suss.
There are sides. Police investigate and charge. Defence lawyers defend.
Police do not cause doubt to be raised about a person's culpability when charges could be imminent. It's like handing the defence a package with a winning argument. That is the issue with what he has said.