Code with no _unit tests_ or _code reviews_ possibly, it was obviously tested by the scientists that it made somewhat sensible and self-consistent predictions.

In addition, predictions by earlier versions of the code had been peer reviewed, which is about as good as it gets outside of computer science.

The report itself wasn't formally peer reviewed either, though in the crisis mentality of March this year not many people had the patience for peer review. Frankly, most still don't if you consider the amount of news about unreviewed science that get published.